Car audio problem….

Sprinter van, 7 year old Teac head unit, stock speakers. Whilst driving a couple of weeks ago the stereo suddenly turned itself up to max volume (never done this before), quickly turned it down but it blew drivers door speaker. So I invested in some Pioneer door speakers and Alpine tweeters along with a Kenwood sub (combined speaker and amp jobbie). When I went to wire it in, the HU didn’t appear to have any rca outs, so I ganged the sub in with the speaker wires. Sounded great for a few minutes then went all distorty at low freq. I’ve since found the multi-plug that plugs into the back of the stereo giving you rca outs, so left the door speakers/tweeters connected straight to the HU and wired from the single rca out subwoofer connection to a splitter to the sub.
Again all sounds good for 10 mins then any song with any bass sounds awful, tried turning the bass on the hu right down, but still the door speakers are distorting.
I have powered the sub from the leisure battery rather than the vehicle battery, but they share a common ground, so I don’t see that being an issue. There’s no separate crossover/filters but the tweeters have capacitors built in to block low freq.
My thoughts are that, as all sounds great for about 10 minutes, the amp in the HU could be damaged/have a dry joint and as things warm up I get issues, am I missing something/doing something wrong?
I could lash in an old Alpine single din HU and see if I get better results, any other ideas?
Cheers
Paul

Is the RCA out, full frequency?
If it’s not a dedicated sub out, and the subs amp has no crossover dial your sub will be being fed full range audio.

Also is it definalty a line out and not a amplified from the head unit?

Do things go back to normal of you unhook the sub?

That said, going back to your initial problem, sounds like the head unit could be faulty?

Hi, it is a dedicated sub o/p, things don’t go back to normal with the sub unplugged.

You need to try another head unit with a basic cross over to at least prove what is wrong.

Try your old Alpine if it is known to be a good one. Doesn’t have to go in the van, just on a seat next to you will be ok.

Saying that, how are you connecting it all up if you have different brand components in the front?

Sounds like early 100W Marshall JCM 2000s on which the PCB board resistance dropped as it heated up which resulted in leakage and bias drift. Sounded fine for a few minutes then more and more overdriven till it blew its valves.