in the Series 3 XJ-6, (1979½ to 1987), that is the classic symptom of the A/C Amplifier failure. (don't confuse this part with a stereo amplifier, it has nothing to do with sound, it is a component of the Delanair MK-II climate system in your car and mine).

This A/C Amplifier is located on the driver's side,near the radio, left side of center console. It has a large, round, cream plastic, male/female connector with many thin wires on both ends of the connector. You need to remove the "right knee" upholstery panel, the one with the black plastic floor vent, to find it in the jungle of wires there. To remove the panel, loosen and remove the two phillips screws at the black plastic vent, and remove the vent, then pull the panel towards the engine and down to the floor.

Thereis animproved A/C Amplifier replacement made in the US. and cheaper too.

go to the following link:

http://www.johnscars.com

get their phone number from the website and call John. This part is not shown in the website.

Be very gentle and patientwhen messing with the wiring in that area, the wires thereare very sensitive and you could create a secondary problem if you start pulling wires too rough.

Jose