You need a lot more information to make an accurate determination. Mainly you need to know what the impedance load you are putting on your amp and what the amp can handle. and you need to make sure there are no other drivers already on that channel (if you don't know what else is wired in).
When you run in series, you can simply add up the impedance of all the drivers: two 4 ohm drivers in series puts an 8 ohm load on the amp - generally not a problem. But you don't get as much output.
In parallel, those same two 4 ohm speakers now put a 2 ohm load on the amp. On most amps, you are getting into a danger zone, especially if you drive the system hard. Calculating the resistance in parallel is a little trickier: it is the reciprocal of the sum of the reciprocals. For example, two four ohm drivers calculation would be
- Reciprocal of 4 is 1/4. 1/4 + 1/4 = 2/4, reduce to 1/2, then take the reciprocal, which is 2 (ohms)
- When they are the same and it is just two, it is easy enough just to halve the impedance of each (4 / 2 = 2)
- It gets trickier when more than 2: three 8 ohm drivers in parallel would be about a 2.67 ohm load: 1/8 + 1/8 + 1/8 = 3/8, reciprocal is 8/3, which converts to 2.6666.... (also trickier when it is two speakers with different resistance)
- Once as an experiment, we took four drivers, created two series paths with each pair, then ran those series routed pairs in parallel. The net load was equal to the load of an individual speaker. Kind of a silly approach, but was fun to do as a teenager (and completely irrelevant to you right now - sorry I digressed)
One other detail to keep in mind is that when you wire two drivers together, they (theoretically) have the same output, so the A Pillar will seem to overpower the door (since it is closer to ear). You may need to introduce a way to balance the levels.
I hadn't caught this before, but you may run into issues running one type of driver (in this case, a tweeter) from an output designed for another one (mid-range). Depending on where the crossover is, you could be attempting to feed a mid-range signal into a driver designed for a different frequency range. Sure, you will get sound out of it, but it will sound terrible.
I haven't looked into how Tesla sets things up, but I would venture to guess that they are running active crossovers (before the amplifiers) as that is the more energy efficient way to handle. If that is the case, I would say that unless you wire drivers to matching drivers (mid-range with mid-range, tweeter with tweeter), you would probably be better off not doing anything.
Just my $0.02
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ETA: You probably know this, but make sure you are phase correct when wiring up multiple speakers:
- Parallel: negatives together and positives together
- Series: negative from speaker 1 to positive to the next one in line.
If they are out of phase, you will experience less output with two speakers than you did with one (the waves are cancelling each other out).