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<E28> Aftermarket Stereo Wiring (No Tunes When Braking)

This is a followup to my earlier posting, where I related a problem 
with a newly installed aftermarket Blaupunkt casette/receiver in my
'87 535is. Several other digester's responded that their 535's also 
lost switched power when the brake lights are actuated.

- -Since the brake lights share the same fused circuit as the factory 
 radio wiring, it seems that there is a voltage drop caused actuation
 of the brake lights. 

- -The problem becomes less frequent after prolonged actuation of the 
 brake lights....suggesting that there is less resistance/voltage drop
 since the brake light-bulb filaments are still warm, hence less voltage
 drop when re-illuminated.

- -The problem seems isolated to the fused circuit shared by the radio and
 brake lights.....actuation of the the ac, blower motor, headlights, etc.
 do not cause a fatal voltage drop.

- -I can only pickup a small deflection on my needle type ohm-meter. I've
 assumed that the voltage drop was at the switched lead since I never 
 loose memory setting when the radio powers off after a voltage drop. 
 But what confuses me is that I thought the brake lights are not under
 ignition/switched power...this would suggest that the shared circuit is
 with the "constant" + wire to the radio rather than the swithed lead

Before using a power relay for the switched power as suggested by Jeff
Wilcox, I would like to try a different circuit on the fusebox. Is there
an unused switched accessory power lead available under the dash. Where
is it located and what color is the lead?

If no sapre pig-tail is available....how is the fusebox panel secured so I can
lift it enough to run a new accessory lead on a different circuit (for example
rear defrost circuit).

Any other suggestions welcome.

TIA,

Jonathan
'87 535is
'72 2002tii