Updated: 11-Jul-2023
Factory Heidelberg Radio Functions
If your car still has the factory Heidelberg radio/cassette head unit but no Operating Instruction manual, the following list outlines its basic functions.
On/Off, Volume knob: Turn the knob clockwise to switch the unit on; continue turning to increase the volume. Turn the knob counterclockwise to decrease volume and to switch the unit off. When powering the unit on/off, a <click> should be heard.
Balance/Treble: While pressing the on/off knob in, turn clockwise or counterclockwise to adjust the sound balance of right-to-left speakers. While pulling the knob out, turn clockwise to increase treble, turn counterclockwise to decrease treble.
Bass control: Turn tab clockwise to increase bass, turn counterclockwise to decrease bass.
Fader control: Adjusts the sound balance of the front and rear speakers.
< > buttons: These are the tuning buttons. When the < button is pressed, the radio frequencies count down; when the > button is pressed, the radio frequencies count up. This is done manually or automatically, depending on where sensitivity selector switch is set.
Sensitivity selector switch: With the switch at the MNL position, radio stations are selected manually. With the switch at the center SEEK position, radio stations are selected automatically, including weaker stations. With the switch at the right SEEK position, only powerful radio stations are selected automatically.
1,2,3,4,5 buttons: Station preset buttons. To store a station, press and hold the desired button. The station will mute for a few seconds; when you hear the station again, release the button. A maximum of 5 FM stations and 5 AM stations can be stored.
AM/FM button: Switches from AM to FM and vice versa.
SCAN button: With the sensitivity selector switch in the MNL position, pressing the SCAN button will automatically search for stations. When a station is found, scanning will stop at that station for 5 seconds. If you like the station, press SCAN again to stop auto searching; if you don't like the station, after 5 seconds the unit will search for the next available station.
METAL button: Press the button in for metal or CrO2 tapes; leave the button out for regular tapes.
Dolby button: Press the button in for tapes recorded by the Dolby Noise-Reduction System in order to eliminate tape hiss; leave the button out for regular tapes.
PRG button: Press this button to change tape play direction.
Anti-Theft Heidelberg Radios
SAFE mode security code instructions: 
Wiring in a New Radio
What size radio will fit? Any single-DIN radio will fit; the only size restriction will be the depth and that is usually a result of a bracket at the back of the radio. Search for radios at BestBuy.com and/or SonicElectronix.com; they will narrow down choices based on your vehicle using their "fit guides".
Modern head units have an accessory-on (ACC) wire, something the Cabriolets do not have. Not connecting the ACC wire can result in the car's battery being drained whenever the unit is not in use. How, then, is the ACC wire connected? Download the guide below for instructions.
Radio wiring guide:
The AKTIV System
An amplified stereo system installed on '91-'93 USA Cabriolets and Euro Cabriolets with Gamma radios. Read all about it by tapping here.
Speakers
Factory speaker sizes
- 1980-1981
- Front: 4x6"
- Rear: none
- 1982-1990
- Front: 4x6"
- Rear: 3½"
- 1991-1993
- Front: 5¼" with 1" tweeters
- Rear: 4x6"
Accessing the rear speakers
Pry off the cover of the window crank, remove the screw and pull the handle off. Then, gently pry the panel off using a long, flat-head screwdriver, or interior trim removal tools.
Upgrading the speakers
Depending on the dimensions and mounting hole locations, some aftermarket replacement speakers fit no problem, while others may take a bit of modifying. Those with pre-1991 Cabriolets can install the speaker grilles, front speakers and door pockets from a 1991+ Cabriolet (but may need to enlarge the speaker hole, depending on speaker magnet size). And of course the speakers can be upgraded to more modern, better sound-producing speakers; most online car audio retailers (listed in the side bar) have compatibility searches. Will you need to install an amp? That's entirely up to you and depends on what type of system you're installing. Installing what are essentially stock replacement speakers do not require an amp, even if you're running new 100w speakers off of the original 45w head unit.
1982-1990 owners: Wanting to upgrade to 4x6 rear speakers? The factory-original speakers from a 1991-1993 Cabriolet will bolt right in (if wanting to use the rear panels and/or speaker grilles, you will also need the extended window crank assemblies... details below). Aftermarket speakers will depend on mounting holes, depth, etc.
Extended window crank assembly parts and part numbers:- Spline/spacer adapter (155847363)
- Gasket (411837447)
- Base (155837595)
- 18mm screw (N0142675)
- Washer (N0121301)
- Crank (321837581, without chrome; 321837581A with chrome)
Replacing the Antenna
You must remove the driver's side wheelhouse liner to gain access to the antenna housing and cable inside the fender to replace the antenna.