Stereo shuts off when turning on the front blower motor?

[Headhunter38]

87 van here. Previous owner installed an aftermarket stereo. When turning on the front blower motor straight to high from off, the stereo shuts off. If I go slow and move it from off to low to the next spot etc, the stereo stays on. Are these circuits connected in some way from factory, or is this likely a screw up from one of the many previous owners?

[Jan-Willem]

Hehe, are you sure the physical mechanism behind
the heater knobs is not ‘touching’ a badly connected wire going to the power of
the stereo?