Sounds like the radio is toast. With that disconnected does the dash now power up and the fuse remain intact?
I know you removed the DSP amp and replaced the radio but I didn't like to suggest it may be a self inflicted fault. You've just got to hope it's in the wiring along the sill shorting to ground and not have to admit you've spent 3 days looking for a problem of your own making. Although it would qualify for the My biggest cock up thread......
Right, so you have a short to ground on that Purple wire. It comes from the plug on the BeCM and then has a splice in it where it splits into a number of feeds. The ETM doesn't say where the splice is but I would suspect it may be under the plastic sill panel, most wiring from the BeCM to the rest of the car lives under there. Once it gets to the dash it then feeds the dash, the radio, clock, ignition key illumination, footwell lamps and also runs to the back to supply power to the DSP amp. I suspect the feed to the DSP amp goes across the car under the dash and then down the sill on the other side.
Nanocom should fire up as soon as it is plugged in, it doesn't need the ignition on. To connect to the BeCM, it needs to be off anyway. If you've got power, maybe you don't have a ground?
In fact, thinking about it, the cover over the sunroof had been left open on the Ascot and when I first got in it I got drenched from the drips of condensation on the inside of the sunroof glass. That would drop down straight into the switchpack.....
Pull the two connectors on the BeCM that the feed from Fuse 1 exits out of and see if the fuse still blows. If it does, the fault is within the BeCM, if it doesn't, plug the one at the rear of the BeCM back in and see if it blows then. If it doesn't the dash should now be working and the fault is within the switchpack.
The Ascot was also covered in mould inside when I started on it a few weeks ago, purely from sitting outside over the winter. Most of it simply wiped off with a damp cloth although I did need to use some proper leather cleaning stuff on the seats.
Looking at the state of that cam and followers, I'm wondering if the chirping squeaking noise wasn't them running dry for some reason?
To check them you have to try to lift the wheel up and down relative to the axle which is why the one man tester can't check them. I've noticed that with the modern boots, they don't last. A couple of years ago I fitted a new steering link and the following year the boots were split. Fortunately, the joints were fine so I just replaced the boots.
There's a timer relay under the front passenger seat that wakes the system up about every 6 hours to level the car. If you take that out once you have parked the car, then it won't wake up. If it has still moved overnight, it's a mechanical/pneumatic problem, if it hasn't for some reason it is thinking it is levelling the system when it actually isn't. Why do you drop it down to low when leaving it?
That's a one man test unit and it will show show up slack in wheel bearings and steering joints but not top and bottom ball joints on a P38. If they've never been changed check them, I bet there's slack in at least the top joint.
Mine drives fine but the Ascot doesn't have the same precision to the steering. Admittedly it is on a pair of well dodgy tyres but the only other difference between the two is that mine has had new top and bottom ball joints. They need to be totally knackered before they'll fail the MoT unless the tester knows what to look for. If you jack up on the axle so the wheel is clear of the ground and use a crowbar under the tyre to try to lift the wheel, if there's any slack, they need doing.
If there's slack between turning the wheel and the steering box doing anything, give the box adjuster a tweak. If you mark the Allen screw before you slacken the locknut, you can see where it was and give it a quarter turn. Don't go too silly or the steering will stiffen up but you can take the slop out if you do it carefully.
Don't forget that things are interconnected. It may well be that if the dash isn't powering up the BeCM will know (as the BeCM and dash talk over a data link) so may inhibit power to the windows switchpack or outstations as it thinks the ignition switched supply isn't there.
The one time I had the SRS light and nothing else, I'd had the instruments out. Even though the ignition was off, when I plugged them back in, some of the warning lights flashed on very briefly and then when the ignition was switched back on, all I got was the SRS light. Turned ignition off, unplugged the main connector, sprayed it with contact cleaner, plugged back it in and all was back to normal. If you've got power at C1276, I'd suggest that is the next thing you try considering the amount of damp you've had in there.
Although there's 4 windows and door outstations, the common thing there is the switchpack, if that has no power then none of them will work. The BeCM is responsible for supplying power to them but if you have power at fuse 1, and on both sides of it, the break is beyond the BeCM. Permanent live power to the switchpack comes from pin 8 (Purple/Blue wire) on C1290 at the BeCM while power to the instruments comes from pin 7 (Purple wire) on C1276. C1290 is a 12 way white connector that is down the side of the BeCM next to the transmission tunnel, while C1276 is the 10 way white connector on the back of the BeCM (the only one there and accessible from the rear passenger footwell. Both of these are fed from fuse 1.
You can easily tell a non-smoker, it wouldn't light a fag if it didn't......
So everything you'd expect other than the dash? In position 1 all you would normally get is windows, radio, wipers and fag lighter socket powered up. Message centre should also show the odometer and If the door is open, door open. I've had nothing on the dash except for the SRS light once before and all that needed was a squirt of contact cleaner in the RH plug into the instrument panel but that wouldn't explain no windows.
Following on from Marty's suggestion, does anything power up with the ignition in position 1? Position 1 on the ignition switch supplies a ground to pin 13 (White/Pink wire) on C1280 which is the 16 way white connector into the BeCM (second one from the right below the BeCM fuse panel). Check that you have that ground signal when ignition switch is in position 1 and 2.
Water in the power board is more likely to cause things to work when you don't want them to rather than the other way round and is highly unlikely to let the engine run. But as the power board is on the top, water ingress would get to the processor board first and nothing would work.
Have you got power at Fuse 1? That is supplied directly from Maxi Fuse 1. Odd that the engine still starts as that requires both relay 15 and 19 to energise so you obviously have an ignition switched supply.