once you do find out what size they are plan on running new wire to them. run them under the carpet to up into the console under the dash. nissan used a one wire common system and modern stereos are much happier with two wires to each speaker. tape off the old wires so they don't short. The wiring for the fronts will work fine even tho the speakers leave a bit to be desired in audio quality. you will need to identify the speaker, power and antenna leads from the schematic in the fsm and then splice in to your stereo plug the correct wires. Nissan did not use DIN standard color wire in those years. if you use crimp butt connectors cut your leads different lengths so you don't wind up with a huge bundle of connectors all in one spot. better to solder and heatshrink the connections. remember in japanese blue wires are marked L on the schematic. you will also need to buy a mounting plate from car toys or crutchfield or make one from a piece of aluminum as i did. crutchfield has an 800 number and usually helpful if you ask nicely. I would also replace the coax from the antenna to the radio as I have had two fail on me (radio shack). I seem to remember the grilles are held on with spring clamps unless you obviously see screws on each corner - either way they are readily removeable no need to pry up one corner. If you do a search you might find a post where some kind soul laid out wire colors in a chart. It is not a daunting task and I have no idea why whatever shop you took it to couldn't do it unless they wanted to sell you a radio and speakers. Pic attached is my 83. that mounting plate was made from a piece of al flatbar 4" x 1/8 thick. I screwed it to the plastic carrier from the old stereo. when you do that you can move the radio over to the left as I am going to use that area for a fuel pressure gage. the bought plates put it smack in the center .