Blaupunkt SC 202 D (C) audio system on Audi 80 B4

Problem: rear bass speakers do not work, treble speakers work.

That’s what you get, when savages repair your car:

Removing autoradio is not that easy. You need small T8H torx (star) screwdriver and four flat 2.4x50mm (2.0x50mm) screwdrivers. Unscrew 4 plugs with T8H torx. Put flat screwdrivers vertically inside with slight angle to push radio locks. Then push flat screwdrivers slightly to the middle pressing radio locks and slowly remove Blaupunkt SC 202: https://www.youtube.com/watch?v=jBY9ci8jxZw - How to remove a car radio | Opel / Vauxhall Astra G 98

8A9 971 733 A - rear speakers cables (RR, RL)

357 035 447 A / 928 525 - brown connector - speakers (middle port).

357 035 447 / 928 524 - black connector - radio power (bottom port, battery icon).

https://www.drive2.ru/parts/vag/357035447a/B-ZLgEAULzQ

SDA - Serial Data Line, SCL - Serial Clock Line, GND - ground, MRQ - Mute Request.

Take care about foam rubber on cables. Old glue destroyed it and if touched, it falls apart. I tested radio outside and cleaned all contacts just in case. When TP (Traffic Program) is activated and you turn off Audi 80 B4, take out ignition key with radio turned on, car beeps. If ignition is turned off, radio will play 55 minutes and turn off. You can turn deck on and listen without ignition key 55 minutes. To balance stereo middle point, pull out switch-volume regulator a little and turn right or left.

Blaupunkt SC 202 sounds great if we mean radio with aerial antenna stretched. I do not have good cassette and can't tell about tape player. There are noise / hiss issues with cheap adapters. Cassette adapter produces too much background noise. Cigarette lighter MP3 FM transmitter plays better but still produces background noise and isn't loud enough. Both adapters are comfortable to listen up to half volume range. Do not use AS (Automatic Scan) and TP (Traffic Program) with FM transmitter.

Modern car radios usually need KL, Masse and 12V+ lines, but Blaupunkt SC 202 needs following battery connection: + to KL pin and - to Masse pin.

Mid range / treble speakers are connected to bass speakers on this audio system. If all treble speakers work, it means that power and cables are ok, but why bass speakers died on rear doors?

Audi 80 B4 rear door panel disassemble

Audi 80 B4 Nokia rear bass speakers repair

Kebidumei Car Cassette Player Tape Adapter from Aliexpress

https://www.allaboutcircuits.com/news/teardown-tuesday-bluetooth-car-fm-transmitter/

https://www.youtube.com/watch?v=lvZPKAInSiw - Blaupunkt SC 202