I have a specific Tow truck installed,it is all in working order and when i spawn it via simple trainer it works......But how do i make a specifically make one particular type of tow truck spawn when requested. What file do i need to edit?

What do you mean? Does it have extras that randomly spawn?

I have a custom tow truck from this website installed. But what i want to do is whenever i call a tow truck via police radio. I only want the custom tow truck to spawn not the default rockstar one.

Their May be an option in the traffic policer settings to change which model spawns. Short of that just install it in the slot that spawns by default