Clock working when light switch not connected but stops working when connected and lights on

sorry for the laymen terms, when I connect the light switch to the dash, the clock turns off when igniation is on and lights on, switch them off and the clock is back on again.
any suggestions on what I can do?
thanks

First thing I can think of is Phantom Grounds or Power.

I would remove the Cluster and on the main connector carefully remove the shroud, lift the mylar and place one strip of electrical tape on the tongue of the connector PIN side.  Replace the mylar and the shroud.

Fine the earthing wire (Brown) and splice a pigtail about 10 inches long to it on the (wire side) of the connector, and a Ring connector to the free end.
Scuff the paint behind the cluster to metal and use a sheet metal screw to attach the ring connector to the scuffed paint area.

As you didn't state the year, then I don't know if you have a digital clock or a analog clock, but refer to the Bentley and in the wiring for your year look for the Clock Voltage feed in, it should be 12V key in or out.

Turn on the lights and it should still be 12V.

If you still are having issues then I would suggest that you need to get to the Battery cables and referb them, that is clean them, then replace the Grounds to the battery and the frame to the engine, as that may be the real cause.

Also Check all your Fuses for continuity, as electricity loves to find new paths to ground or sources of power if its local (fuse) isn't good….

I have seen starters use speedometer cables for grounds and that is a smoking good thing to melt clusters.

I have also seen sensor grounds being used for Starters and or headlights and that usually fry's the ECU….

I know it's just a ground or a fuse, that can't be your issue can it? muahahahahahahah

9 times out of 10, this is the cluster earth. on the MFA cars this runs to the side of the head, all you need to do is clean or repair this connection and problem solved. look for a brown/white wire