Just had a look
It is not a hiss problem it is a ignition problem so what I am saying is that it is not the hiss cutting in and out it is in the ignition barrel wiring

For the rr-y model
Just looked at the wiring diagram and the 3 wires off the back of the barrel run to
1 Fusebox
2 Rectifier/regulator
3 Fan
No. 2 is the primary power lead I think

On the rr-2 and rr-3 there are only 2 wires with nothing going to the fan
Check to see that its not just a loose wire at the bottom of the barrel and if you do have to solder something remove the hiss collar and the key as they don't like heat

If it is the loose key get a key cut for the lock and tape the old key to the transponder so it still reads it, it has to be reasonably close for the hiss to read it

best of luck