X2 on what Harry said above. Grounds are responsible for most of the electrical troubles on these cars. Make sure the pot metal tail light housings have a good ground. Use a jumper wire if your not sure.
Also, a local FGF'er went through a whole bunch of trouble shooting when his signal & tail lights were doing crazy things. Someone suggested changing the tail light bulbs out one at a time. Sure enough one of the brand new bulbs had an internal short.
Since your problem started after the work in the trunk I would try removing the tail light bulbs one by one and see if you can at least rule out what does / doesn't help.
