MG Car Repair: wiring, engine mountings, wiring loom

Question
I can get my lights and markers to work the starter turns the engine over and I have spark. But that's about it When I checked the connection wires I found that both leads are hot. therre's 12 volts on the groung lead and when I turn on the switch the other lead has 12 volts?????

Answer
You have a wiring fault.  

The first thing I would check is the grounding wire for the engine.  Depending on the year, it could be anywhere.  I have seen it across one of the front rubber engine mountings,  Underneath by the gearbox mounting, further forward using a bellhousing bolt, and even from the alternator mounting bolt direct to the bodywork.  

If there are no side lights, check both of the top 2 fuses, but if there are no headlights, that is more likely to be the actual light switch.  They were cheaply made and often fail.  Be careful with the light switch, the brown wire is permanently live with no fuse, and the dashboard is an excellent earth.  It is a recipe for a wiring loom fire, so disconnect the battery before you start.