To be certain, you should:
1. Consult your owner's manual: This is the best source for the exact location in your specific model.
2. Check the fuse box diagrams: The diagrams will often show the location of the flasher relay.
3. Look for a small, square or rectangular black box: Flasher relays are usually easily identifiable by their size and shape. It often has wires connected to it.
If you can't find it after checking these places, a search online for "1991 Chevy pickup flasher relay location [your trim level/body style]" might yield more specific results.