ok i only have tail lights if i tap my brakes after i turn on my lights but if i use my brakes i only have the third light for brake lights and no tail lights turn signal flashes fast if either brakes or head lights are applyed at the same time everything except the tail lights work by themselves normally ive re grounded them already and have replaced all the switches ( brake turn and head light ) and it makes no difference nothing works like its suppoed to
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I have seen this happen when the wrong bulbs are used and when the tail light assembly is shorted internally. Also if you have a trailer wiring harness, eliminate it all together before testing.
Generally when the turn signal flashes fast it is an indication of a blown bulb or WRONG BULB.
The brake light bulb and turn signal bulb are one in the same for the rear.
Use this 1994 GMC 1500 Exterior Lighting Wiring Diagram to help guide you if you need to trace the wiring for a possible short or open circuit.