A wire running between the plugs does not mean they are wired in series. It is probably providing 12 volts across each of the plugs, meaning they are wired in parallel. That copper wire is providing roughly the same voltage to each GP, and then the engine block is taking the electricity away (the ground).

People often remove the salt shaker GP indicator when wiring for parallel, but that didn't occur in your car. So instead of each plug getting 12 V, they are each getting somewhat less than 12 V. That might explain the long warm up time despite having plugs in parallel (the normally faster setup).

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).