Does anyone know or have experience of the bulb control units and if the various issue numbers are backward compatible?
In particular :-

86 - 928 641 603 05
84 & 85 - 928 641 603 04
83 - 928 641 603 03

Thanks in advance.
Roger
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Roger, I found a (minor) reference to the BCU on the CD's. Doesn't mention later years.

Lifted From pg 31 "Electrical Troubleshooting and Repair 928S":
Bulb Monitoring Unit
The bulb monitoring unit (behind CEB) has been revised. The monitoring of electric power for the tail lights is now accomplished with electronic components and no longer with mechanical reed switches, The new bulb tester (928.641.603.04) is fully interchangeable with earlier models.
Landsheer
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There are different bulb controllers, here is an overview:

928.641.603.02 used in 1978-1982 and 1983-1986 (reed relay current detection)

928.641.603.04 used is 1983-1986 models (electronic current sensing, backwards compatible)

928.641.603.05 used in  USA 1983-1986 (? no idea where it differs from others)

928.641.603.06 (? no idea where used and where it differs from others. Porsche does not list it)

928.641.603.07 (? no idea where used and where it differs from others. Porsche does not list it)

928.641.603.09 used in 1987-1995 models.

Regards, Theo

1992 928GTS midnight blue