well, everything depends on what do you want. Being stereo you have the front speaker and rear speaker on driver side. They are "left" and "right" respectivelly.
SO, options are:
-you can just feed the front speaker as the Mono setup was made without any extra option... matching green and black wires between Radio and Dash harness. Violet wire will keep unused. You can replace either of both plugs and match them. Change the 3 ways plug on the dash harness with a two ways one, and the violet wire will be hanging around OR replace the two ways plug on the radio pigtail for the 3 ways one, and install green and black wires on the cavities to match the dash harness. One cavity will remain empty on this.
The rear speaker will not be used on this.
EDITING: I think the stereo harness section can be removed from dash harness and left it at a side and just use the portion running to front speaker to connect straight the radio to the FRONT speaker
OR:
You can get a rear fader knob and harness to use the rear speaker correctly as a dual speaker setup on Mono radio. This was an option on some Mono cars ( AM and AM/FM ). The rear fader control goes into the map light/seatbelt light panel on the cluster "eyebrow" area of the dash.
diagrams related to these setups
you can also bridge the green and violet wires together on dash harness to feed the rear speaker along with frontg, but won't have a pan/balance volume control between both.
OR:
Get the correct radio stereo for your car. They are not hard to find really ALTHOUGHT they use to be into $150-250 rate. If the AM/FM Mono radio you have works, could be sold into around same price rate. It worths more the AM/FM option, than the Mono or Stereo option
