Why wont my reverse lights work on my 1989 chevy 1500 all my fuses are good

Check the switch that is under the dash on the brake pedal arm.

When truck is what?

Check fuses first. Then the brakelight switch then the bulbs. But the curcuit board in the tail light has a good chance to be bad. They burn the board out a lot in the GM trucks.

YES THERE IS A FUSE, BUT THERE IS ALSO A SWITCH MOUNTED ON THE TRNSMISSION, USUALLY THE WHERE GEAR SHIFTER HOOKS. HAS A SMALL LINKAGE ROD FROM SHIFTER LINKAGE TO SWITCH

change out your distributor cap and rotor button

If the fuse is good and the reverse lights are good, I would suspect your problem will be the reverse light switch itself. I believe yours is located on the side of the transmission where your linkage connects, if you have an automatic transmission.
If you have a manual transmission , you'll probably have to contact the local automotive parts store and ask them to tell you where it's located.

The electrical switch attached to the shifter on the transmission has quite a few functions in that.
One of those functions is the back up light switch

could be bad headlight switch

It could be your drive selector is not alligned correctly. Is it a automatic and does the needle lineup with the R? IF NOT, REMOVE BOTTOM COVER PLATE AND TRY TIGHTENING THE LINE.

Yes it screws in on right side of tramission and you will need a 7/8 inch deep socket to remove it. You may be able to clean the old one once it is removed. There is a little ball thatgets dirty and may you can get by with cleaning it. Good Luck. Ed