'90 t-bird shake(vibrates) when headlights come

I have a '90 t-bird and I have noticed that when I turn on the headlights or the rear defroster the car begins to shake a little. Before the the headlights or the rear defroster is on the car is smooth you cant feel any vibrations of the engine. But as soon as the headlights or rear refroster comes on the car shakes( vibrates). Anybody have or had this problem? Any info would be helpful thanks.

look to ground wires in trunk for fuel pump and rear lights

Check to see if your alternator is putting out enough to keep up with the extra strain of the power being pulled. The 3.8 I used to have did that with the headlights and it turned out The pulley was loose and with the extra strain of the lights it would vibrate. No biggie hope to have been some help!!!!!!

I checked the alternator by disconnecting the positive wire from the battery while it was running. From what I heard if the car turned off when the positive wire is disconnected then the alternator is not putting out enough. When I tried that my car kept running so I dont think that is the problem. The pulley might be, I will check that out. Thanks for the info.

Hardly any alternator will keep an engine running at idle....they don't put out enough amps.....You have to hold the throttle open a little to keep the RPM's up to check it like that...