Just the right side speakers work in my truck

If you haven't already checked the balance on your steep do that if its ok then take right hand wires off off amp & concert them to right side if they work it's the amp if not you have a short in wiring on right side of car pull door cards off then make sure no wires are bare or touching together or earthing on body

Determining the Cause of the Problem Let's assume our system has a head unit, a crossover, a two channel amplifier and two door speakers with only the right speaker working (the left speaker is not working). If the door speakers are behind the door panels then we want to make sure the speaker is the problem before removing the panel. We'll work backwards starting with the amplifier and ending at the head unit. Testing for a Speaker Problem With the vehicle and the stereo turned off disconnect the speaker wires for both the left and right speakers at the amplifier. Now connect the left speaker to the right amplifier output (which we know works). Turn on the stereo system and make sure the balance is set to the middle position.If the speaker still doesn't work then the problem is somewhere between the amplifier and the speaker Testing for an Amplifier Problem Now we'll need to see if the amplifier channel is bad. Put the left speaker back on the left amplifier channel and the right speaker on the right amplifier channel. This returns the speaker wiring to its original configuration. Again, with the vehicle and stereo off unplug the RCA cables from the amplifier's input and switch them so the left cable is in the right input and vice versa. Turn on the stereo system. Testing for an RCA Cable Problem For any intermediate components between the amplifier and the head unit follow these steps for each one. We only have a crossover so we'll just do this once. Begin by testing the RCA cables between the intermediate component and the amplifier. Unplug both RCA cables from the intermediate component's output. Now plug the working channel's RCA cable into the non-working side of the crossover. In our case the working channel is the right so plug the right RCA cable from the amplifier into the left side of the crossover. Turn on the stereo system.
If the right channel continues to work then you know the problem is in the RCA cables between the amp and the crossover. esting for a Crossover or Equalizer Problem At the input of the intermediate component (in our case the crossover) flip the RCA cables around (left to right input and vice versa). Turn on the stereo system.
If the right channel continues to work and the left channel continues to not work then you know the problem is in the intermediate component. Replace or repair this component.
If the problem is now in the right speaker and the left speaker works then the problem lies before the crossover. Turn off the stereo system and return the RCA cables to their normal places.
Testing for a Head Unit or RCA Cable Problem Pull out the head unit from the dash so you can access the RCA cables. Unplug both RCA cables from the head unit. Do not disconnect any other wiring. Plug the right RCA cable into the left channel of the head unit. Turn on the stereo system.

so many things that can be wrong. First step is to take the working speaker from the right, and plug it in on the left. If it works, replace the bad spkr. If not, it's a wiring/electrical problem, and will need more testing..

Check to see if any right speaker wires are pinched which would cause a short circuit. Make sure no bare right speaker wires are touching the frame of the car.

check your wiring

Re wire the other side from the amplifier

pull the rubber boot back off the wiring harness in the crease of the drivers door and check for broken wires there. common problem due to the number of times these wires flex to allow the door to open and close

Check the fuses. The left and right sides have seperate fuses for the lights. Not sure about the speaker, that might just be a coincidence that it doesnt work. I reacently worked on a hyundai that had the same problem.

Check the ground connections for the speakers.

The first step is to make sure the "balance" on the sound system is centered and not shifted all the way to the right. Contact me on FixYa if I can be of any more assistance. Thank you.