starter relay

"no current" ? It is hard to measure current. Do you mean "0 V, rel. to ground"? As stated, the yellow wire is "crank request" from the key switch. Next step, move the blk multimeter lead on BATT- terminal and measure again. If ~12 V that time, then the case of your relay is not grounded to BATT-, so no coil current can flow. If the starter relay actuates, you will hear a faint click, which should apply +12 V to the thicker brown wire that goes down to actuate the starter's solenoid (really big relay which clunks as its big contacts engage and the starter pinion gear moves out).

But, somewhere in the starter relay's coil path there must be a neutral safety switch. Perhaps they put that in the path of the yellow wire within the cabin in some cars. Most have it on the coil- side, as a wire down to the NSS on the transmission (shorts to gnd in P or N).