I'm posting his photo because some browsers are unable to get to the photo...
Based on this photo, I'm guessing it was the MAF or MAP sensor that ignited the HHO. I really don't understand how a spark plug can possibly ignite the hydrogen inside the intake manifold. It's just impossible. Intake valves are CLOSED when the spark plug fires... It's that simple... The only way this could have happened with a spark plug would be a backfire out the intake. That's not likely because this is a fuel injected vehicle. Backfires are not common on EFI vehicles unless you've got your ignition timing REALLY screwed up or you have an intake valve that's stuck open. If you had a stuck intake valve and were getting backfires you would have known about it earlier and would probably have a damaged Throttle body. Now that I'm thinking about it you probably have a damaged throttle body anyway because of this explosion.
If my theory is correct: The MAF/MAP sensor ignited the HHO, then all you really have to do (if you know you have HHO in your intake) is to disconnect the MAF/MAS sensors and crank up your engine and let it run for a few seconds. It will run like crap but it will flush out all the gas in your intake manifold. It might even idle really high for a moment or two while the engine uses up the high volume of HHO gas. You will know when the HHO is used up because the engine idle will return to normal. You can then turn off the engine, plug your MAF/MAP sensor wires back in. You will then need to disconnect your battery wires and touch them together for at least 15 seconds. This will clear your error codes in your ECU caused by disconnected MAF/MAP sensors. Take my advice with a grain of salt. I believe this should work but ultimately it's your money...
The best use of your money would be to wire it correctly the first time.
Here it is...