I'm no expert on your car - if you don't have wiring diagrams a bit of a search in google ("W124 wiring diagram") came up with a thread on this forum
W124 wiring diagrams
There were also the following hits on BenzWorld
http://www.benzworld.org/forums/w124-e-ce-d-td-class/1336430-w124-230e-1988-c-wiring-diagram.html
And MBclubuk
W124 wiring diagrams - MBClub UK
They might have the right one for you. Who knows the rear fog light might be on the same fuse as the buzzer - it kind of makes sense... but I don't know for sure - hence the multi links to diagrams...
This could be a red herring though.
The first thing I would do is check under the dash for anything obvious => meaning find where the earth points are and see if they are corroded. And if you let the buzzer buzz you might even find it this way - who knows!
As for your boot not closing properly (most people here call it a trunk by the way!) look at lubricating the mechanism and check the condition of the rubbers. You might find it works a bit better after a clean and lube. Otherwise you will probably find there is a little bit of adjustment on the catch. Does your car have a boot release lever by the driver's seat? If so clean and lubricate that end too.