Tweeters

That doesn't sound right. It should be able to run off speaker level inputs and only need a hu switched live. The power cabe direct fused off the battery. Even if it only takes line level inputs you can get adapters from speaker level. Really though there's nothing to feed back to the hu to make it die.

I know this is an old thread, but I just came across it.

It is possible to destroy/damage the head unit, connecting a sub, if you don't do it right, especially if you're using a speaker-to-line-level adapter.

Here's how - most head units use a "balanced" output where neither side of the speaker connection can be grounded, the RCA line level connector is "unbalanced" so one side is grounded, and a "special cable" is required between the two, otherwise the RCA connector on the subwoofer amp will ground one side of the speaker and that shorts half the output stage in the head unit.

This "special cable" comes with the Pioneer under-seat subs, it is in effect a speaker level to line level adapter as Lambert describes, and I'll just mention here that not all of these adapters are properly designed, but the Pioneer ones are, and they work well in a Jimny.

Both of my Jimnys have Pioneer under-seat subs, both connected to the OEM head unit via speaker-level-to-line-level adapters, the three door has the Pioneer provided "special" cable, but when I did the five door, I used a sub that I had pulled from another vehicle, and I'd lost the cable, so I built my own, essentially a copy of the Pioneer one.

It was whilst researching the "adapter designs" that I discovered many of them just address the required reduction in voltage level, but not the isolation from ground.