Probing the transistor / heat sink proves nothing
"What it want's"
Assuming this is a 4 wire, you have two circuit paths in the system
You start out with switched 12V from the ignition switch. This is a problem. Forty year old wiring means there can be voltage drop at the connections. So start with that.
Turn the key on with engine off. Probe the ballast resistor with your meter until you get the highest reading. This should be the switched 12V. What you want to do is determine if this circuit has a voltage drop problem
There are two ways to do this
A.......Measure carefully at the ballast to the engine block and note the reading.
Then measure carefully directly at the battery posts and note that reading. There should not be more than .3V (three tenths of one volt) difference between the two
B......Second way to check. Determin the high side of the ballast as mentioned earlier. Leave your meter probe on that terminal. Now move your ground probe over to the POSITIVE battery post, and adjust the meter to read low DC volts. This should read voltage drop directly, and again, you are hoping for .3V OR LESS
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Now, check the ground path the same way. With the meter set on low DC volts and the key in "run," stab one probe directly into the top of the battery NEG post. Stab the other probe into the orange CASE of the ECU. NOT the heat sink, the grounded case. The lower the reading the better. It should be zero or nearly so. More than .1V (1 tenth of a volt) is too much
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Now, check for voltage at the ECU. You'll have to have a wiring diagram. There is one wire, assuming this is a 4 wire box, that provides power to the box. Best way is to stick a pin through the top of the connector to the correct pin. This checks voltage "under load." Once again, hook the other probe to battery POS and once again, you are looking for a low reading, the lower the better.
Simplified diagram:
Wire at far top right is switched ignition voltage from the key
With key in "run" you can check coil + voltage to ground, almost meaningless. This will vary from say, 5V to say, 10-11V depending
You can check coil NEG terminal to ground With key in run, this should be very low, perhaps less than 1 volt. This tells you that the coil is drawing current through the box. High voltage (12V) here means the box is not turning the coil on. Might be a bad box
Voltage at heat sink / transistor. Meaningless. I believe this is same as coil NEG
MAKE ABSOLUTELY CERTAIN that the firewall, back of the ECU is scraped clean, and that it is mounted clean and TIGHT with star lock washers. Does not hurt to daisy chain a no 12 or so wire between voltage reg, ECU, and battery NEG or engine block
