I've got a vintage blaupunkt Frankfurt deck. quite often (but not always) I'll get a pop through stereo system when I hit the brake pedal. sometimes (but even less) it will happen with the blinker, though quieter when this happens.

these aren't on the same circuit, what could be causing this? and why doesn't it always happen if its a problem?

Dead voltage regulating capacitor on the input of the Blaupunkt unit?

Pressing the brakes or turning on the indicators will affect your voltage ever so slightly. (Probably more at low revs than high revs)
If the voltage regulating/smoothing capacitor is dead then I guess you could get that effect....

Going through and cleaning up all your terminals might help too.

Most of the 914 radios also had a built-in or wired-in suppressor to minimize the effects from th engine sparkplugs/dizzy/coil firing (usually a staticy buzz on the radio), but it would also suppress these power surge pops too.

These suppressors were common on radios up to the 1970s-80's.

You might see if you can get one from Radio Shack or other local old school radio/electronics store & wire in-line to see if it helps reception & sound overall. They're usually only a few bucks & some time under the dash.


Tom
///////

Electrolytic capacitors will go bad as the years go by. In my experience, 20 years is the rough start for problems.