I see what you mean. I didn't notice that 121 was just a piece of wire. Anyway, you could splice into 39 but a splice has to be done correctly and isn't the best for your immediate question. Assuming the aftermarket volt meter has stud terminals with nuts, you would run 39 to it using a crimped-on ring tongue terminal. Then another piece of 39, with another ring tongue placed onto the same voltmeter stud, to Tach. A jumper, in other words. Avoiding a splice.
