That's my post.
After thinking I'd lost my 500's faulty powered mount, I found it in my new workshop, at the bottom of a cardboard box I hadn't looked in for years.
I took the faulty mount apart with intent to fix it, and found corrosion on the main board's ribbon cable connector, and on the ribbon cable itself (see attached pics).
To get access to both ends of the ribbon cable, you need to:
1) Remove the small Phillips screw holding the magnetic sealing plug/rubber strip at the bottom of the mount.
2) Use a thin-bladed knife or Stanley knife to carefully prise the rubber strip off the bottom of the mount (it's held on with very thin double-sided tape).
3) With the rubber strip off, remove the two small Phillips screws that are now exposed.
4) With all three screws removed, you need to prise the lower plastic sealing cap off the bottom of the mount. This job is very tricky to do without damaging the mount or the cap; the cap is held on with silicone-type adhesive.
With the sealing cap off, you'll see where the other end of the ribbon cable goes into the block holding the 24-pin connector.
I have a powered car mount I have no intent to use, and considered trying to pilfer the cable from that.
Sadly, no go - the car mount and bike mount use completely different ribbon cables.
Seeing as how Garmin no longer provide any support for the 400 / 450 / 500 / 550s, my best course of action will be to clean the corroded cable connector and cable with a good electrical contact cleaner.
If I can't salvage the current ribbon cable, I'm going to try finding a generic cable that can work as a substitute. My local computer store might be able to help.
Failing both of those options, I'll finally have to admit defeat.
