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Nice write up. Except that the 201 doesn't use PWM blower control. It uses an old fashioned resistor bank, located behind the battery. So nice try, but irrelevant information. It's a non-model specific forum , so misunderstandings happen. Your question is obscure and model specific, don't be misled.
Power for the blower in an '87 190 is full time through the ignition switch. The two relays that are built into the switch (idiotic design, engineered like no other car) are grounded by the head unit and switch power to the blower circuits. One is turned on by switching to defrost, and it powers the blower at high speed. The other is turned on whenever the head unit is turned on, and powers the three speed function. The head unit operates the relays by simply switching the grounds on and off. Inside the head unit, I don't think the electronics are involved at all, but rather this is done by mechanical switching.
Two possibilities here:
1) some sort of problem inside the head unit, probably a failing switch, is allowing it to ground the relays
2) One of the relays is nearly bridged with carbon, and the switch needs to be cleaned or replaced.
If the CC system is otherwise working well, I'd permanently ground the relay that allows 1-2-3 operation. better yet, just bridge the stupid relay and be done with a totally useless complication. That would keep the blower available at all times, and would take the head unit out of the equation.
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