Hi Nathan, I am no record specialist, but here is a few tips.
At first, you say that you hooked it up to a vintage Pioneer 850 tuner? I suspect that you maybe hooked it up to a SX-850 receiver. If it is a TUNER you are connecting it to you will never get sound out of it. You need to connect your record player to a receiver/amplifier with a phono input.
I guess that you have connected the record player to the phono input and connected the smaller cable(ground wire) to GND (ground) on your receiver.
Finding were to put the wires on the headshell can be difficult. Is it a original headshell for the marantz? If yes, I suspect it is the same headshell I have on my 6100. On my original headshell for the 6100 the cables are placed(considering it is placed on the tonearm):
Top left: white
Top right: red
Bottom left: blue
Bottom right: green.
There is a lot of things to adjust on a TT but probably the problem with slipping needle/cartridge is needle pressure. You can adjust the needle pressure at the back of the tonearm. You have one weight and a scale. The scale is often "lose" and that is because you can adjust it regarding to what cartridge/weight you are using. Unplug the TT, place the tonearm over the platter. Screw the weight on the back of the tonearm until you get it completely in line horizontal. It should not go up och or down, it should be exact parallell to the platter/record. When you have acheived this, rotate the scale and set it to zero (0). Now you have 0 grams of pressure on the needle. I don't know which cartridge you are using, check the specs for your cartridge and follow the recommendations for needle pressure. To set the right needle pressure hold both the scale and weight and adjust it to the right pressure(for instance 2 grams).
You can often use approx the same value to the anti-skating, set it to the same value as the needle pressure. (place on the right of the tonearm).
Now try to listen again, if it still doesn't sound, we will take it from there.
Best regards
Filip