When doing replacement or upgrading on 1985 – 1988 GMC Brigadier with 2 4 headlamps, the bulb size guide is necessary for you to take the reference.
Jump to corresponding car model with the catalog bellow.

Low Beam Headlight 1988 H4656
1985 – 1987 H5006
High Beam Headlight 1988 H4651
1985 – 1987 H5001
Back Up Light 1985 – 1988 1156
Brake 1985 – 1988 1157
Front Side Marker 1985 – 1988 1157
Front Turn Signal 1985 – 1988 1157
Parking Light 1985 – 1988 1157
Rear Side Marker 1985 – 1988 1157
Rear Turn Signal 1985 – 1988 1157
Tail Light 1985 – 1988 1157
High Beam and Low Beam Headlight 1985 – 1988 6014
Back Up Light 1985 – 1988 1156
Brake 1985 – 1988 1157
Front Side Marker 1985 – 1988 1157
Front Turn Signal 1985 – 1988 1157
Parking Light 1985 – 1988 1157
Rear Side Marker 1985 – 1988 1157
Rear Turn Signal 1985 – 1988 1157
Tail Light 1985 – 1988 1157