Hi, 1998 P38 2.5 DSE, tailgate started to have erratic behavior some months ago. Sometimes it opened, sometimes only but the upper half, but many times it didn't open.
Now it doesn't work at all, no signs of life.
Rear button rust free and working (it gives contact). Driver door lock mechanism (LHD) is brand new, fob working as it should thanks to Martin's magic filter, all other locks working trouble-free. Fuse on BECM is ok, interior lights working (I guess it's the same fuse).
And now many questions:
Any idea?
What should I check first? Is there any electric measure I can do? And where?

PS is the tailgate lock linked to right front door lock also on LHD p38s?

Yes, tailgate is linked from the RH front door latch, irrespective of what side the driver sits on. Power comes from F15 which powers lots of other things so you'd know if that had blown. Power travels on a Purple wire into a connector behind the trim on the RH side of the boot. From there all wires are white (a security thing) so it is easy to find the right one, wires on one side are different colours and all are white on the other side. In the same connector you have a Green/Red wire which goes back to the RH door latch and should have a ground on it when the RH front door is unlocked. Power goes to the tailgate latch, through that to the pushbutton and the other side of the pushbutton goes to the ground wire. So when you push the button in, power goes through the latch (to release it) via the button to ground from the door latch.

If you can get the lower tailgate carpet trim off, you can test at both the latch and the switch.

i have been having the same issue with my rear latch , sometimes it works and others it doesn't . still trying to work out if it's the button or the solenoid,. if i operate it manually, move the levers in the tailgate by hand that is , it starts to work again for a short time then stops again. i need to fix it .

Thanks Gilbert.
Trim is already off (I've got a simple piece of string connected to the lock. Lesson learned after the first not-opening). Today I will check if a ground is coming to the switch button with open doors.
If I don't find the ground on the button I can assume a broken cable from the door or from the tailgate or a broken switch in the right door lock...
But if there is power (and ground) on solenoid I think I should investigate the tailgate lock..

Well, I've got ground from the door lock to button and from button to solenoid.
But I don't have +12V on the solenoid.
I'm going to check purple wire on the side of the boot.

Hi, problem solved. Was the simplest and most obvious hypothesis: a broken cable in the rubber hose. Those white cables are very thin and quite rigid.
Maybe in the future I will change them all with something better (and with logic colors).
For the moment have repaired them with tinning and heat-shrink tubing.

Was the rubber split or cut? One way of breaking into a P38 is to pull the rubber down between the tailgate and bumper and then use a sharp knife blade to put a ground on one wire at a time until you find the one that comes from the door latch (which is why they are all one colour to make it harder to find the correct one). It's also another way of opening the tailgate if the door latch is faulty. That would put a break in the insulation so weaken the wires and allow the conductors to corrode.

Happy to hear that you found the problem.

@Gilbert rubber had a longitudinal break, but I don't think anyone cut it. It had been reinforced with insulating tape, the wires were all close to the breaking point and the positive wire to the lock had broken gradually. I guess was mechanical wear of the cables and rubber, the old owners probably used the hatch a lot, I think they had dogs..
But who knowns what happened in 25 years, your hypothesis is not to be discarded.

And now ready for another adventure.. it's time to find out how to stop water leakage from the top corners of the top half of the tailgate seal

My intermittent tailgate button was down to a cracked connector clip on the end of the loom. (Also have tailgate top corner drips to sort).

so today i fixed my tailgate , i hope . replaced the push button , still didn't work properly , fiddled with the plug out of the button , got an unstable reading with the multi meter so i replaced the plug to the button , still didn't work properly . only thing left is the solenoid , knowing that i didn't have another solenoid i unscrewed the cover on the solenoid and done an inspection , played with the motor and the plug , came to the conclusion it's the motor in the solenoid , knowing that the earlier model solenoid is different i wasn't happy as i may have to buy a new one i decided to dismantle the earlier model solenoid with a couple of screw drivers as it is clipped together and removed the motor that looked very similar as the later one i tried to fit it to find that there is some small differences . FYI the motor is identical it just had a small metal shroud that i removed easily and on the front of the motor there is a small piece of plastic that just unclips and then it's the same as the later model. so the early solenoid motor is the same on the inside as the later model solenoid for those that have a 1997 model rover that is different on the outside only , the mechanicals are different but the motor will fit.

Glad to hear you got it sorted. And for free!