Sounds like you know the fault is in the kick panel, where there is a connector for the satnav coax
Who knows? Kick the kick panel hard enough and some force will make it as far as the radio and/or roof aerial.
Anywho. Some progress tonight. Managed to get the sat-nav into Diagnostic mode. No camera, so pinching Aaron's idea, display shows...
12/00 HDP0.0 DG 13:06 A S0.0
N50:50:11. W02:38:35 H+0100 R0
N0- 0 N0- 0 N0- 0 N0- 0
N0- 0 N0- 0 N0- 0 N0- 0
Which is apparantly somewhere in Ludlow. So next step, lift the gearstick gaitor, defeat the parking lock, and start another attempt at extraction.
I modified my Halfords dual din extractors as shown in one of the other posts. after mich wiggling and jiggling it finally came out. Then the problem with the cheapo extractors was obvious. They splay out too much, such that by the time they get far enough in to engage the clips, they actually miss. So note to anyone using these things - make sure you bend them so that the two arms are parallel when they go through the hole in the front panel.
Next step, peer into the back of the radio cage. It all looked pretty good to me - seen much worse on aircraft. So I decided to buzz check the wiring from the Din cage to the kick panel. Both inner and screen OK, and virtually infinate resistance between them.
Next up, something those in the know won't like - but i figured that if it's goosed already then to hell with it - i measured the input impedance of the satnav aerial terminal on the back of the radio - measured 10K ohms ish.
So I plugged the radio back in (partly to listen to the Simon Mayo Confession on Radio 2), and measured the impedance between the core and screen of the cable to the radio in the footwell. Measured minus 6K ohms ish. Funny measurement, but the radio was powered on, and as long as it isn't zero or infinate, then hey-ho.
So then measured the impedance of the cable from the footwell to the roof aerial. Ah-haa. Zero ohms - well 0.03 ohms once I'd changes scales on the DVM.
Next step - disconnect the roof aerial and repeat the test. Again still measured 0.03 ohms at the footwell. Aditionally, measured 0.01 ohms at the roof end.
So - It looks like I've got a short in the coax cable between the roof and the footwell - most likely at the roof end. Bugga. I'll have a trawl through the RS and Farnell catalogues at work tomorrow, but these sorts of small RF connectors usually require expensive crimp tools to assemble which we won't have. Anyone ever replaced this cable?
The other trick I know about is to take a fully charged car battery, connect the negative to the screen and the positive to the core. Wherever the cable catches fire - that's where the short is
if you'r lucky, it'll clear the short. If you're unluck, your car burns to the ground