The test point is not hard to find, it is just hard to use on LH2.0. There's a two-terminal socket coming from the harness near the AMM and ignition coil with a pink and blue/white wire. The pink one is the mixture test point; the blue/white is one you can ground to disable the idle air motor. Bentley probably tells you to use an LED tool or dwell meter. The LED tool won't work right on LH2.0 but works as advertised if you have an 85-88 LH2.2. I haven't actually tried a dwell meter, but mine is ancient and I suspect the method to be flawed because the oxygen sensor is pretty slow at idle. Probably the best way to use the test point without building an adapter is using an analog meter to watch the transitions between about zero and 3.5V. Ideal mixture is set when the meter spends equal time at both places.

I suspect yours will be stuck at one reading and not move. I also predict you won't be able to learn anything about your AMM by taking the resistance readings recommended in the Bentley-- the one they claim measures the hot wire just reads a fixed film resistor that has about zero chance of failing. Your best test is the one I suggested: Put it back in the car, peel back the boot on the AMM connector, find pin 7 (white/red) and pin 9 (orange). Turn on the ignition, measure pin 9 -- ensure you have battery voltage- around 12.5 -- engine not running. With the motor off, measure voltage at pin 7. If you have less than 1.5V, start looking for another AMM, you won't be able to compensate with the mixture adjust pot. When you find another, repeat this test. If the voltage is higher, make sure the engine coolant temp sensor is OK.
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Art Benstein near Baltimore