Got a new one on the '99
Last night, I went for a drive and within about 30 seconds of setting out, with lights on, I got 3 messages:
RH Tail Bulb out, LH Tail Bulb out, License plate lamp bulb out.
Upon inspection, all 3 lights were merrily shining away. Shut off and upon re starting, got the same thing.
I have checked RAVE thinking it was possibly an earth issue. Found that they do not share a common Earth point, so non starter there.
Any clues as to where I look from here?

You don't have any simple faults do you? The RH tail light and number plate lights both come off C1283 at the BeCM but on different pins whereas the LH tail light comes off a different connector even (C1291) so there is nothing common there and, as you say, the ground connections are different too. Are you sure nobody hasn't sawn the back of the car off?

I've known the bulb failed warnings to come up if the bulbholder has a bit of corrosion so the bulb doesn't illuminate instantly but not for 3 to all do it at the same time. The other thing to bear in mind is that the bulb failed warning will come up if that individual feed is drawing too little (bulb blown) or too much (bulb short circuited) current. I found that out when I wanted to run a 21W warning beacon on a wide trailer so connected it to one of the tail light feeds (on UK towing electrics the two tail light circuits are fed separately, left and right). After a few miles, the beacon went out and the bulb blown warning came up on the dash as it was drawing more current then expected. I found that if I switched the lights off and back on again, it would reset until next time it gave the warning. It appears that in that case, once it has given the warning it was switching that circuit off to prevent any wiring being burnt out.

Was going to attack this issue this morning, but it seems to have miraculously healed itself.....For now......

Of course, this is the '99 with the randomly wonky fuel gauge as well.....That seems stable for the moment too......