Another option might be to find the smallest amplifier you can find (Walmart or Best Buy). Get an adapter cable to go from headphone jack to Mono-in to amplifier. Run a speaker wire from output of amp to your dash speaker.
I don't know what size your dash speaker is, but on my '71 C-10, it had a 4x10 mono speaker in the dash. I found an option that had two 3.5" speakers mounted into a plate that was cut to the standard 4x10 speaker shape. If you did that, you could get a simple Headphone to RCA adapter cable to feed the signal to the small amp, then run right and left channels to the new twin-speakered dash speaker giving you stereo sound. Bet you could get all of that for around $100.
