Electrical question '86 300SDL

This last weekend, while attempting to replace a defective license plate lamp (the right one) I discovered that there is no voltage present at the lamp socket. I took the trim piece off the trunk sill and looked behind the carpet. Access to the lamp/lock assembly appears to be fairly straightforward. My question, however, is whether or not this is the best place to begin looking for the problem. The lamp fault indicator on the dash is on when the headlamps are on.

Secondly, I took this car up to 14,000 feet this last weekend in an attempt to escape the intense heat of the city. I noticed that, at idle only, the engine sounded significantly more 'pingy' than at 5200 feet where it tends to make a more muted purring sound. Why is this? I never noticed any untoward behaviour at 14000 feet such as excessive smoke, but it did take just a second longer than the usual 1 second it takes to start the engine.
Lastly, once beyond some incredible turbo lag (about 4 seconds) this car pulls up hills at altitude unlike anything I have ever driven. What a monster! Why would turbo lag be so pronounced at high altitude? Any comments would be appreciated.