The original poster had me confused with a reference to grounding the steering column switch--not a factor if not using a relay. The setup I've used on my '69 144S came from a '68 145 parts car and I had first adapted it to a '68 144S using the M41 D type that came out of the '68 145. That system used a 3 prong relay with the column switch providing ground for the relay. After running the D type for as number of years I switched to a J type and then eventually switched the trans to the current '69. In the 1967-68 system power to the relay is provided by fuse #5. Fuses 1-5 are on a bus (if that's the right term) connected to terminal #15 on the ignition switch. On the 1969 fuses 1-4 are "bused" and connected to terminal #15 at the ignition switch with a short jump wire to fuse #5 (where the OD switch gets power). In an original 1969 there is no relay for the OD. I used the 67-68 system with the relay. This is all taken from a Clymer repair manual.
According to my wire diagrams for 1970 power goes to the OD switch from terminal #4--but that one is bused to #3 which gets power from terminal #15 at the ignition switch. Is that all clear? :-)
SO--on your '68 you should be able to connect to any fuse from 1-5 which are all powered from the ignition switch terminal #15. -- happy shifting -- Dave