It sounds to me like you have the CC that was put on by dealers between '89 and '91 due to the recall.
http://www.recallrecord.com/auto/volkswagen/vanagon/
Volkswagen VANAGON vehicle speed control recall
Recall Date:    Jun. 8, 1989
Recalled Component:     Vehicle speed control. 
Manufactured by: Volkswagen of America,inc 
Date of Manufacturing: From Aug. 1, 1987 to Dec. 1, 1988
Units Affected: 300
Defect Summary: When using the cruise control the accelerator cable tension may allow the cable to sag and become entangled with the water pipe bracket impeding its return to idle position.
Consequence:    Failure to return to idle position could lead to anaccident.
How to Fix:     The present accelerator cable bracket will be replaced.
Recall Notes:   System: cruise control. Vehicle description: van camper type vehicle.
Vehicles covered in this recall:	
        � 1987 Volkswagen Camper
        � 1988 Volkswagen Camper
        � 1989 Volkswagen Camper
        � 1987 Volkswagen Syncro
        � 1988 Volkswagen Syncro
        � ...

I am currently running into a problem while trying to add a VSS device, that plugs into the speedometer
in that hole in the back of it. I've found it (VSS) won't work and ultimately found that the speedometer
housing, which is white plastic, does not have some parts that allow this kind of unit to work. It was a dealer
replacement speedo, due to the cruis control recall.

In order to get the VSS I'm installing to work, the housing has been replaced with an OE/stock unit, and now
the speed sensor works.

bob
 
On Jun 6, 2013, at 9:22 AM, Jeremy Stovin <jjstov@YAHOO.COM> wrote:

he arm has a black thick wire (probably bundle of wires inside) that goes up and around to a black box. The black box had an old label that said something to the affect: "AT Magnet...Servo..." Memory is hazy.