every combi i've worked on the prox switches have indicator lights on them around the circumfrence of the back end where the harness plugs into them. They look like orange dots, sometimes you have to look closely at them and maybe even use a mirror to get close to them to see it, but if there is alot of light, the ones that are enclosed in a dark area like underneath the cabin or inside the wheel well mechanism at the front wheel you can usually see.

But if none of them are lighting up you have a problem somewhere, most likely a lost ground or a bad relay. He may want to check the directional switch, they have been known to give problems like this too. Once he sees all the connections on the directional switch he'll understand what i'm saying.

And trev your not wrong about the pressures generated when these wheels/cylinders max out at their endpoints, they do trigger diverter valves and check valves in the system to insure proper hydraulic operation sequence but the sensors control the electrical part of these functions. If they are not working properly theres a 100% chance there will be malfunction of steering/travel.

Sensors at the wheels tell the hydraulic system what position the wheels are in so that the valves that control the travel will divert the fluid properly for the mode and direction the machine is trying to move in. If any one of them stops working you may get movement but the hydraulic system will cross circuits and cause a 'deadhead' scenario and keep the machine from moving correctly. In some cases i've seen it keep the truck from moving at all or when you choose a direction it moves in a different one that selected all because of these sensors.

Thats the fun part about dealing with electrical over hydraulic controls ;o)