A bit of an update on my Focal component installation. I found the sound was not pleasant coming from the tweeters - too much high end overall and tiring to listen to. I realised that with no crossover, the tweeter was being driven by the full dynamic range rather than just filling out the detail at the top end. The only way to make it bearable on a long journey was to set the high end to the minimum on the equaliser.
I tried some Focal crossovers but I had the same problem I’ve read about elsewhere, which is that some crossovers upset the NIT which then shuts off the sound completely.
Fortunately I’d kept the old tweeters and noticed the 2.2 microfarad capacitor which acts as a high pass filter.
I removed it and soldered it to the Focal tweeter, in line on the positive terminal (any auto electricians probably want to look away now). I glued the capacitor to the speaker mounting.
I’ve tried it out and it sounds much better. I’ve got the equaliser set back to flat.
No doubt if I knew anything about what I was doing I’d have sorted this out a long time ago, but I thought my experience might help someone else.
I found this helpful page with a bit more information about capacitors as high pass filters.
Use this JavaScript calculator to determine what value of capacitors you need to use for a crossover design.
carstereo.com