When truck gets warm the headlights start blinking on and off what could cause this?also where is the headlight relay located for my 1990 dodgr w-150?

Check your charging system:

https://www.askmehelpdesk.com/cars-t...ml#post1973613

There may be a problem with the alternator (worn brushes) or voltage regulator. I suspect that the alternator needs rebuilding or replacing. Disconnect the negative battery cable, prior to working on the charging system.