The MEP002 I have sitting here has a hot wire going to the manifold heater, a jumper from the manifold heater to the glow plug by the fan and another jumper from that glow plug to the other glow plug. It appears to be all original as it was manufactured. The manifold heater and both glow plugs are on the same circuit hooked in series. Each one, the manifold heater and both glow plugs do not have their own power source, but a single one to all three. If a resister is used for a power reduction, it could be connected in-line after the manifold heater leaving it 24V and before the first glow plug making both glow plugs 12V, at least on this one. I have two others on another property and are the same.
I see what you are saying, the glow plugs are parallel but it is the power source that is in series, it does not have an individual source going to each heater/glow plug. I guess it is how you are looking at it
, whether upside-down or backwards. Didn't mean to sound confusing
The grid heater is nothing more than a manifold heater, which it already has.