if you know you have a prob with the stat change it! if its blocking the system, it will build up pressure and cause more serious engine problems e.g. blown head gasket. It shouldnt require head removal to change a stat.
back to the idea of running a manual switch though (its a usefull thing to have when sat in traffic) you need to locate the fan switch (which is normally on the back of the rad) find the wire that goes controls slow speed, chop this wire and join an additional wire into it (so you have 2 wires going to the slow speed connection), then solder and sleave it. Run this wire into the cabin and to a switch. run another wire from the battery (make sure itsd fused!) and run this also into the cabin to the switch and hey presto you have a manual control over the fan, aswell as the original slow speed function.
