First, choose a new radio that physically fits. Either Ford stock radio from any year, or a fancy new aftermarket radio. Then get the wiring diagram for this new radio's input/output connectors. Crutchfield offers them or just Google for them.
Secondly, you need to find the Ford wiring diagram for your particular (1991?) stock radio, which tells you which pins do what within your existing radio connectors.
Buy a generic $10 adapter wire harness connector that plugs into your truck's existing dashboard radio connectors (like those Logan03CO posted). This avoids a hack job. Each wire has a specific purpose, per wiring diagrams, so start matching up each old wire to each new wire. Splice those together. That's it.
For example, see my photos here:
http://imgur.com/a/xWGY6
Do you know how to solder, and to use heat-shrink tubing? Otherwise, crimps?