[R129 SL] electrical ???

[George Jenner]

Look for a wire shorting out when you move the shift lever.  You will 
probably find this wire beneath the wood panel on top of the transmission. 
The cause of the short circuit may be that a wire has come loose from a 
connector or that the shift lever has, over time, rubbed off the insulation 
so that when the shift lever is moved to exactly the right (in this case 
wrong) spot, the wire is shorted out.

George