Part of the problem is the cost associated with aftermarket head units for these vehicles. You can't just throw in any double-din radio and be done with it, because you have to relocate the climate control, hazards, and OEM radio controls as well as the small screen, which is all tied together. Furthermore, in my case for my Base 2013 Ford Police Interceptor Utility, the base model has tweeters and subs in the front doors but no crossover - meaning that the frequency split is done inside the radio. There are just many layers to upgrading the radio, even installing LOCs is difficult due to the split frequency outputs, so you have to trace the wires for the rear doors and figure something out with that, and rewire the front speakers altogether using crossovers. It's much easier to get an aftermarket head unit, but it's gonna cost ya $1000 and up for that alone. Most vehicles that get audio upgrades don't have these intricacies. Head unit, better speakers, maybe an external amp and an additional amp/sub combo. With these the biggest hiccup is getting the signal from the radio without losing the functionality of your dashboard.
Tl;dr: the system is complicated, and aftermarket solutions are expensive and often unreliable/lose functionality over OEM.
As for me, once I have more money to play with, I'll be doing the following:
1. LOC spliced into the rear right/left channels (spliced in behind the head unit right near the plug; this will be the starting point of all my following steps) since those are full-range speakers and they get the full frequency range of music.
2. Amplifier. Thinking of options... I really wanna do a Dolby Pro Logic setup with a center speaker and an external sub, but if I find that that's not doable, then I'll go with a 4 channel amp... Just front left/right, rear left/right. Police package already has some wiring done, but it's easy enough to run wires for power for the amp using the access hole in the firewall anyway.
3. Use factory wiring and make crossovers for the front doors.
- crossovers will be near the head unis, since the wires are split for the tweeters and woofers for the front doors at the head unit. This will also mean that I don't need to really waterproof them since they won't be in the doors and potentially exposed to the elements.
- I'll make crossovers that match the aftermarket tweeters and woofers I install. Haven't decided on exactly what I'm going to get yet.
- Use factory wiring, no need to run wires, nice and clean setup is the result with the crossovers hidden behind the dash. *Chef's kiss*
4. Install the aftermarket cones in the doors (and center of the dash if I'm going for the 5.1 Dolby Pro Logic setup). Again, no plan for specific speakers and tweeters yet.
5 (optional). Install external subwoofer and subwoofer amp if applicable. This will be the only thing I really need to do some serious running of wires for, but luckily the Police Interceptor Utility has a lot of features that help with that, such as the rear power center behind the right-rear cargo area wall. There is a powerful positive and negative terminal there meant to run police equipment like radios and lights and such.
And I believe that's everything I'd have to do. I'd lose the ability to fade front or back, but I'd retain the factory head unit functions of volume and balance left/right at least, as well as the factory equalizer. I listen to music on my phone exclusively via AUX input anyway so I'll use my phone for more detailed EQ if needed. I believe the line output converter will allow the factory volume controls to work... I've never done any of this before though so let me know if that's incorrect.
If I missed any important steps here or if I'm plain wrong about anything please just let me know! I'm super excited to eventually do this and I'd love to have a more refined and specific plan drawn up before doing this stuff. I like to be thorough.