Check your ground wires on the light make sure they are tight back there where the lights are.. and make sure the bulbs are tight in the socket ..The year of this vehicle is not old enough for it to be a big problem.. unless someone tampered with the wiring ..
Testing the fuel system ,fuel pressure an flow , voltage drop testing the electrical . Hooking up a factory scanner an checking engine mangagement sensor data . Doing these thing's is the way to finding the problem . Not guessing an looking for common problem's .Their are no magical fixes for auto repair . Viewing wiring diagram's an getting out the testing equipment , DVOM - digital volt ohm meter , scanner , lab scope etc... Your best bet ,take it to a qualified repair shop .
Having a factory or professional type scan tool hooked up checking DTC'S - diagnostic trouble codes in all the modules on the vehicle . Insturment cluster , PCM , BCM ,ABS , SRS etc.... Should be the first step in the diagnostic procedure.. Could have a serial data network problem . Do you know what a serial data buss is ? In the case of your vehicle it is GM class 2 data buss . Anytime a bunch of thing's electrical wierd out , suspect the buss.Buick , chevy , cadillac , olds , pontiac all used this . Buick Class 2 Operation Diagnosis 1998
Try looking at the fuel filter, it might be partly clogged or just clogged.Locate the fuel filter, disconnect it and (I know this sounds strange) blow through it, if you get resistance of any kind, then it's bad, if it blows completely through with NO resistance at all, then it's good.
on the end of the splines in a groove of the shaft there should be a round "c" clip ( like a round piece of spring wire ) that is used to retain the shaft in the diff
it compresses into the groove when removing or installing
make sure that the shaft is the correct length for that side and get the clip from a drive shaft shop / service center
Could be the light switch is bad , but that would be just guessing ? And you don't want to replace parts form a guess posted on here !
The way to find the problem is with electrical testing , using a volt meter , wiring diagrams an leaning how to do automotive electrical testing , how to read an use wiring diagrams etc.. Testing B+ voltage to the headlamps an the ground circuit using voltage drop testing . That is if you want it fixed right . Electric Testing Techniques You Need to Know Elec Lesson 6 AC Diag 1 How to read an electrical diagram Lesson 1