I am upgrading the sound system in my 79. I'll have a Pioneer head unit, 4 speakers-2 in the door, 2 in the front section of the rear panels (next to the rear seat). These will be driven by a RF amp mounted over the rear wheel well on one side. I will have another amp the other side to drive 2 10" RF subs. They are currently in 2 truck boxes, and the last time I had a Bronco I had basically the same subs behind the back seat, facing rearward.

What I want to do is install the subs in the back sectins of the rear side panels. I know they are not strong enough for mounting, so I was going to make a faceplate that would screw into the steel behind the panel at the top and bottom. I'll have to measure to see if 3/4 plywood or MDF will space the speaker out far enough to keep it off the body side panel. If not I can space it out as necessary. Either way, I'd be cutting a hole in the interior panel so the subs will sit as flush to the sides as possible.

The big question is if this will provide adequate sound quality. It wouldn't be a sealed enclosure for each side, so do I need to get some free-air subs, or will these be fine as I described mounting them?

I'm not looking for a hear-my-bass-a-mile-away system, but I do like having a good, full-range system, primarily for classic through modern rock.

 

I might also, instead of a different amp for the subs, run one amp for each side, with the sub bridged across the rear channels, speakers for that side off the front, and the front and rear filters and gains on the amps set accordingly for each side. I could either run 2 stereo RCA cables, one to each side, because the amps will run all 4 outputs off 1 set of inputs. Or, if it would be better, split the sub pre-outs at the head unit and run 2 more RCA cables to each side and have front stereo inputs going to the front outputs, and the rear sub inputs going to the bridged subs on the rear outputs.

The reason I was looking at using one amp for speakers and the second for subs is because the head unit has front L/R, rear L/R, and sub L/R pre-outs. This would require 2 RCA cables to one side, and 1 to the sub amp, which would then go to 2-channel mode.

I could also at a later point add 4 tweeters front and rear and run them of the head unit's internal amp....if needed.

 

I would use one amp for the subs unless there was a power problem. like you said it will give you control over the level of the subs and make wiring slightly easier. I can't think the distance difference between the wires going to each side will cause a phase issue, but use as large a wire as you can (depending on the power of the amp there will be a minimum.)

For the sub mounting. The back side of the subs will probably cause a lot of sympathetic resonance. Even if you don't seal it up, you will need to convert a lot of the acoustic energy. Packing the holes will help considerably, fiberglass insulation, poly fill from the craft store, carpet, anything to slow the sound waves down and vibrate quietly.

You really should try to seal up the cavity, because of the constantly changing air pressure, you may pick up a whistle. you don't have to go crazy with it, acrylic expanding foam, silicone caulk, any thing that can fill the gaps. I would get a Dynamat or eqv bulk pack and use it to both dampen the resonant freq of the sheet metal and seal the cracks. Then pack the hole full of poly fill, about 1.5 lbs of poly fill per 1CF of space.

 

heres another idea for the sub mounting that will allow you to use a sealed or ported box depending on what you want to build. I did this a bunch of years ago so I do not have measurements any longer but it should be easy to figure out.

 

I was thinking of something similar. I may end up just installing a switch at the sub amp for the turn-on wire and using banana plugs into a terminal plate on the side panel. This way, when I need the extra space, I can turn the switch off, unplug the box, and take it out.