Most of the manufactures in semi truck amp applications are using speaker level signal input to source the amps input. The regular speaker output from any radio will work. The only thing needed is to trigger the amp on. On the newer models 2005 -up are using the BT ISI DIN harness and the blue wire with a white trace is used to turn the factory amp on. So hook it to the similar color on your new radio install.
Older models will sometimes use a seperate wire to do this. Color is not as important as it will vary. It is usually the only one left to hook after all the speakers are hooked and power hook up is finished you might notice one wire left and there is no sound but radio shows working. If this occurs and you are sure the speakers are working and radio is wired correctly. I have used a old common sense trick to actually test a trigger wire. Use a inginition wire like the red one and add a pigtail to it with a 1 amp fuse inline. If you hook it to a good trigger wire for the amp it will trigger the amp on. If it is hooked to a non trigger wire the fuse will usually blow. All of this test should be done after you are sure all of the other connectins are done right.