It could also easily be the wiring. I would use a voltmeter and test to make sure there is a signal getting there. Then, if there isn't check on the inside of the truck for a signal. I had a similar problem with my back passenger speaker not working. Where the rubber boot that carries the wires for the door meets the door jamb, there should be a plug. Unplug it and check that the wires to that point are getting power. Luckily I checked that before going any further because i had power there, so I knew it had to be between that plug and the speaker. When I pulled the rubber boot back I noticed half of the wires had been pinched and cut. I just soldered in little jumpers and heat shrinked around them individually and closed everything up and it worked like a charm. In fact it may be easier to try that first, just unplug the adapter and check there before having to take the door panel and everything off. Then you'll know if it's the speaker or wiring in the door, or something from there to the radio.
Victor