steer switches either side of steer wheels are notorious for causing drive problems

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These old girls can be a can of worms, lots of people don't realise there is an EV100 controller in the counterweight, remove the small grey panel with CROWN on it and check the TB1-TB6 screw terminals are secure. I've had these terminals crack on the solder join to the board and cause similar problems, jack BOTH drive wheels up, get an operator to operate the machine and wiggle the TB1-TB6 wires and see if you get traction, or tap the panel with the handle of an insulated screwdriver. Also remove and reseat the PCZ connector and the other two 4 or six wire plugs (name alludes me).

Also, there are two sets of direction contactors, one for left motor and one for right. Do all initial faultfinding with steer wheels in the straight ahead position so both motors operate in the same direction, on full lock the inner wheel turns in the opposite direction to the outer wheel to improve turning and prevent motor stall. If only one set of contactors is operating you need to find out why (operators normally complain of slow traction if so).

Check the handbrake switch if no contactors are operating, you can also press the brake override button as this overrides the handbrake switch to allow for hill-starts and the like. No contactors could also be the start switch by the speed pot. Contactors in but no drive could be the speed pot itself or a connection between it and the EV controller.