I had a similar issue on a Mercury Sable recently. The starter seemed like it was acting sporadic and I had just replaced it not long ago. I checked all of the fuses and everything looked good, I started to suspect a bad ground when I went to pop the trunk and I could hear the actuator struggling to kick in.
I ended up just buying a new main ground strap and installing it and it fixed the problem. Might be a little more of an issue with a Disco because it looks like a web of wires coming off of that ground strap...That would be my first guess though, especially if you can't pop the gas tank, it's a ground issue for sure, if it's not starting it's probably your main.
good to know. I will have to look at the manuel but is the main ground strap connected to the battery? When it comes to electrical work, I am out of my element. Brakes, rotors, alternator, etc. no problem, electrical work I am not as up to speed on.
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