I just picked up the Bosch upgrade kit for my 71 220D, from series to parallel. Each glow plug DOES connect from one to the next with a common wire, not an individual wire going from the relay to each plug. I haven't installed the kit yet (that's for this weekend).
I am dissapointed that they tell you its a kit that includes everything you need, but you also have to get a 6mm line to run from the relay to the battery, a 6mm line to run from the relay to the lead glow plug, and come up with a way to connect these lines to the relay (found some nuts and washers at a hardware store).
The box promises 3-7 second glows and after glowing. Sure beats the 60 second ritual I have with my series plugs every morning.