I've just been working on a friend's Zumo 500/550 that wouldn't charge from the bike cradle & hope the following info might be helpful to others:
1. symptom = zumo not charging on bike mount, but charges ok from mains (usb) and car mount. Therefore zumo unit = ok; bike mount = goosed.
2. check external voltage using the HIDDEN INFO+DIAGNOSTICS SCREENS in the zumo (start zumo, wait until settled, then press+hold the time display at top RHS of screen for about 10 seconds until the hidden info/diagnostics screen appears. the reading to look for is 'Vexternal'; also handy if you want to know your bike's voltage!).
3. ok so confirmed it's the bike mount cradle thats bust. a quick check with the zumo in another bike cradle as a double check just to be sure. yep, definitely the bike cradle.
4. so, a look on fleabay & replacement mounts seem to be anywhere between 40 and 60 quid

but I've a broken one here so nothing to lose so may as well have a look inside to see if anything can be done.....
5. have a read on a few forums / sites & it seems this is a common problem, with moisture getting inside & a bit of corrosion.
6. the back of the cradle comes off fairly easy, just 4 small screws.
7. at the bottom of the cradle, there's a very small ribbon cable that connects the main circuit board to the zumo pin connector. it looks like a few pins are corroded. the ribbon cable is a clip fit into this socket.... just ease back the grey clamp a tadge & the grip on the cable is released. the ribbon cable can now be pulled out of the socket.
8. looking through the cable against a strong light, the small wire tracks can be seen. in my case, it looks like one or 2 are broken (looks like this was due to corrosion), but these are way too small to even think about repairing.
9. took off the small plate at the bottom of the unit to get to the bit that connects to the bottom of the pins on the cradle..... carefully peel off the rubber bit attached to the pin protective cap.... then there's 2 small screws to be removed. then very gently pry off the bottom cover (it's stuck on with silicone sealant). be gentle but careful, it will come off. underneath is another connector at the bottom end of the ribbon cable.
10. this bottom connector is well sealed with silicone, so gently scrape this off with a knife. then (just as before), the grey clamp can be eased back on the connector block & the bottom end of the ribbon cable released (noting which end & side of the cable !).
11. check out the ribbon cable against the light for broken tracks. I found a couple broken (probably due to corrosion & electrolosys). gently scrape & clean up the cable connectors.
12. I re-installed the ribbon cable IN REVERSE, so the wire tracks that were broken (probably the ones that supplied power to the zumo) were now at the other side of the connectors.
13.

it works !! power is now back on via the cradle to the zumo. the broken tracks don't seem to have had any effect (probably because the zumo cradle is far too complex for most users, having USB links etc) but the main thing is that it's charging again, & all the functions needed are now working.
14. better put it back together again, sealing the joints etc back up with silicone. oh & just to be sure, stick a plastic bag over the mount if it's left out in the wet, as it's not up to the 'waterproof' tag garmin would have us believe !
15.

now for a beer !