Best high speed : 1 relay after the switch Reduce overall current flowing in the old circuit : 1 relay before the switch Best of both worlds : 3 relays after the switch Normally you put the relay after the switch in order to get rid of the resistance of the switch (and flaky connectors...). So you would get more voltage if you put the relay(s) after the switch. That's three relays if you want to secure the 3 speeds, but there is not much to be gained in "relaying" the lower speeds. If you look at the spal catalogue p.72 they specify this model incorporates a 3/4 speed resistor (presumably bobbin of nichrome wire) which purpose is to raise the resistance for the lower speeds. My guess, is that the resistance gained through the relay will be marginal compared to the built-in resistor. For sure if you put the relay before the switch you loose a bit of high speed (as some voltage is lost in the switch), but at low speed the current would not pass through the old fuse/relay box, which is the bottleneck of these cars. Regardless on your final layout (before/after switch, single/triple relays) The issue from then on is where to obtain the 12v for the relay(s): maybe you can tap voltage directly from the ammeter as this is as close as it can be from the battery and alternator. You would have to add a dedicated fuse as you are bypassing the fusebox. Maybe a 15A fuse with automatic rearm properly located behind the instrument cluster?