Hand throttle is your answer. Purely mechanical, no wires, no computers, no funny stuff.
I sent this to another member but I'll post here for posterity, its specific to my 1977 with two heater levers and no stock hand throttle, but I'm sure it is applicable to many more cars:
Below are the parts I ordered:
1 x 901-424-037-00-OEM
1 x 901-424-317-00-M100
1 x 911-424-735-04-OEM
3 x 911-424-737-00-OEM
1 x 911-424-792-00-M253
1 x 999-523-006-00-OEM
1 x N-011-524-27-OEM
1 x N01-243-51-OEM
1 x 911-424-519-02-OEM (this is part 17-1 I reference later)
I've attached an annotated portion of the Porsche PET for 1974-1977 showing the handbrake assembly and the parts number. The stuff in the red circle is what you're using/building. The blue "X" are parts you don't need (#16, the spacer wasn't needed for my car- a 1977 but you might need it, I don't know) and finally, the stuff circled in green is NLA.
Part 23-2 is something that you'll need but it's also NLA. This is the part I had to make myself. Luckily, you already have one on the other side by the heater levers so you can make a copy from that. It's not all that hard with an angle grinder or metal saw and some pliers... I'm sure you could model one up and have it 3D printed or fabricated by SendCutSend but that seems like so much more work...
The other part that you need that's NLA is 17-1. This is also needed. Now, Pelican and Stoddard sell the part "911 424 519 00" but though it looks right, it's too short by about 1cm and that makes a huge difference; it works fine for card with one lever for heat control but not two. I cut off one of the threaded ends (on the right) and welded on a threaded metric bolt (after first cutting the head off the bolt) and that gave me the extra length I needed. If you have a mill and a lathe you could just make one of the bolts out of solid stock but I don't have a lathe and welding was a 5 min job. Again, this could be made by a machine shop but it was so much quicker to just weld it up.
Here's the link for the throttle rod collar:
https://www.thingiverse.com/thing:5447656
I printed it out of steel, aluminum and carbon reinforced nylon using Shapeways. The steel was not accurate enough, the aluminum was great but I lost it! and the nylon was what I ended up using. The opening for the throttle rod needed to be filed open a bit (like the thickness of two sheets of paper) but it's doing just fine. My design uses two set screws as opposed to Porsche's one... I figured why not double insurance. Those will have to be tapped. I ordered the screws and tap from McMaster-Carr:
https://www.mcmaster.com/catalog/128/3371
https://www.mcmaster.com/catalog/128/2793
Installation is a PITA and I did it with the seats in place which was stupid, but I'm a glutton for punishment; remove the driver's seat!
Be careful to not accidentally detach the heat levers or the parking brake as that will make your life much less pleasant...trust me, I know.
Best of luck and enjoy the hand throttle- it's super cool and also poor man's cruise control
. (actually, don't use it for that, the parts can probably handle it but after you install one you'll understand why the notion of replacing it due to wear is a scary one)
