Hi Dave Jones @davejones
“A little knowledge is a dangerous thing” – just got both the parts books out and the w/shop manual too..

Mine is a 1987 Model VIN 139052+ and it is indeed different to your 1985 which as you say has ECU in the boot same as all the V12’s do.. I thought all the 3.6l Cabs had ECU in footwell but obviously incorrect.

I should have kept quiet as many items are different on my late model 3.6 cabriolet which was why I bought it originally, though I particularly liked the veneered ski slope.
Mine has it under passenger toe-board next to slot for the cruise ECU. FYI the std fit Jag/Clarion radio cassette won’t have a separate power amp but happy to be proved wrong!

If you have already had your ECU off and cleaned the contacts then that’s really all I was suggesting. It might just work especially if car has stood unused at some point. Mine had cobwebs!

kind regards, Robin
PS I’ll try and check parts manuals for vin changes before I make assumptions next time..