Hi Will,

What model Volvo radio head unit do you have? (I guess factory original?).

The head unit is the version that powers the dash mount speakers for imaging, and through a five- or six-pin DIN cable connect the radio head unit with the amplifier. There is a few different models of each and am uncertain what 1990 740 Turbo comes equipped with. I'll guess the premium model system made by Alpine (head unit, amp) and now in need of replacement Dynaudio dash, door(s), and rear hat shelf or dunno on wagon.

Hum - do you have LED headlights? Audio hum may mean spurious RF emission usually caused by, that also causes hums, back conductivity at ground connections directly or related to the circuit.

Faulty connection with or in the aerial. Grounds? Anything loose?

Badly distorted sound? Your switching diode replacement worked and you tested the amp outside your 740? You've tested the circuits and no low voltage beyond normal voltage drop? The capacitors in the Alpine and older Mitsubishi amps can bulge, leak, cause problems.

Do you and your Volvo 740 reside in a humid place like maritime or riparian zone? Salt with snow with moar salt? You've tested your charging system? The DC output is correct? The alternator diode rectifier pack is okay?

Else, I dunno for now.
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