I bought a used 2000 f-250 last spring and it has a viper remote start (older 2 button model) and keyless entry when i first got it, starter worked. worked once about a month ago
now starter doesn't work. door locks work fine, when i try it, chime sounds,light on dash light like key is on but starter doesn't engage.
i don't know anything about these things and i have no manuel. i think i'm operating it correctly (holding both buttons). maybe someone can point me in the right direction?

 

There is typically a rats nest of fuses tied to the brain. I'd start there, and move to relays next. It sounds like it's mostly working, just not tripping the starter motor.