The schematic for the defroster relay can be confusing. It is not the simplest around...

Here is a schematic drawn by someone and it may help understand what happens.

R6 and R7 are the heaters inside the side mirrors, R4 and R5 are the two sections of the rear window heater (one for normal power, one for extra boost)

According to the manual the functionalities are as follow

Turn “ Push “ Action
Off “ Off “ off, that was an easy one
On “ Off “ low power
Off “ On “ full power for about 15 minutes
On “ On “ full power for about 15 minutes then low power
As long as defogger resistances are powered, the light in the button lights up at full brightness

There are two relays inside relay I-II
Both relays off, power off

When r1 is activated by the electronics, both resistances are in parallel, read full power.
R4 via: bus 30 > fuse 19 > relay I-II 30 > relay I-II r1 > relay I-II 87 > T12 > R4 > T13 > relay I-II 87b > relay I-II r2 > relay I-II 31 > ground
R5 via: bus 30 > fuse 19 > relay I-II 30 > relay I-II r1 > relay I-II 87 > T12 > R5 > MP VI

When r2 is activated by the electronics, both resistances are in series, read low power.
R4 via: bus 30 > fuse 19 > relay I-II 30 > relay I-II r2 > relay I-II 87b > T13 > R4 > R5 > MP VI

When both relays are activated only R5 will generate heat since R4 is shorted by both relays r1 and r2. So I don't think this a normal operation case.


The mirrors are always powered, full power when r1 is activated, low power when r2 is activated since in series with R4.
Pin 50 deactivate the defroster when the starter is activated.
Maybe a bit too late for the OP, but maybe helpful for others

WATCH OUT:
All above connector number & pins are for a '89 car!
You have an '86, so for sure numbers are different!
But principle is the same.

MCS51
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Terminal 50 of the starter is the solenoid, this is a very low DC impedance to ground (inductor) driven by the starter relay. All the time that the starter relay is INACTIVE that 50 connection is a pretty good ground for a relay coil that only requires <200mA to operate. When the starter is activated it is not - so the Blower fan and the X relay will be deactivated - which reduces the system load while starting (that is the whole point of using this as the ground).

Alan
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Up to '89 both relays X and fresh air blower are connected directly to ground, only the defogger is shut down when starting the car.
'90: relays X, Fresh air blower and defogger via starter coil
'91 onward: relays X and defogger only
MCS51
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