O.K. so I stopped for a minute and looked at the schematic for the typical late 60's early 70's GM 8 track.
Some details, for the curious.
This is a schematic and service pages to the early 70's 8-track
1. First, be aware that the output section of the 8 track power amplifier is built around the standard GM DS-503 Germanium transistor equipped class A transformer coupled output section...same as any GM AM or AM/FM radio from the era. Nothing special about this.
2. Second, it looks to me like there may not actually be any need for external "load resistors", either. In fact, it appears
to me that the Front/Rear fader actually provides a nominal load function. It is a 100 ohm 2 watt unit, which appears to be in the circuit at all times. Set it to the middle and forget it is a definite possibility. Appears to me that the output section will always have at least a 50 ohm load present, even without speakers connected. But if you are really paranoid, you easily can lift the .68 ohm fusible resistor (found at 14 and 23 on the schematic), disconnecting the current and completely shutting down those two DS-503 output transistors, (as I previously suggested)
3. Yes, the preamp level signal definitely appears at the output wiper of the stereo volume control. Tap it there, you're golden. Wires #12 and #21. Should be no DC present there, but check with a meter to be safe. The capacitors C30 and C34 block it downstream, and C9 and C13 block it upstream, so no DC present on the wiper of the volume control. Should be able to swing close to a volt there, which is plenty to drive any reasonable head unit aux input.
Just clarifying my previous post, for future researchers who face this same question, and might want to approach it as I suggested.