75 Duster electrical boxes

Now I am familiar with the standard ignition box and the voltage regulator, but I'm working on a 75 Duster that has a blue box on the passenger fender and a black box on the passenger side of the firewall. Both of these boxes have three wires going to them. Any idea what they are? The picture I've included does not focus on them both, but one is visible in the top left of the picture. I've seen these before in friends' junk piles, but never thought about them because I've never seen them in use. And is there a difference between blue and black? Can I interchange them should I ever need to replace them? Or are these not anything important? This is a car that I'm buying from a friend and I'm slowly working on it in his driveway so I don't have to tow it home. There's also a couple of relays on the driver wheel well that I'm not sure what those are for; the starter relay is on the firewall. Would this have the seat belt interlock that I've heard about? There's also something on the speedometer cable which I believe to be a signal generator for cruise control, but there's no sign that this car ever had cruise. I'll try to get some better pictures next time I'm out working on it. Thanks for any input.

You really need a service manual. I have a 76 "somewhere" I'll see if I can find it. Part of the info you need 'may' be in the 73 shop manual which you can download from MyMopar for free

Also, I posted the later wiring section in the manuals thread.......Some of it might be in there

Free service manuals

Here is the wiring we scanned for the 76 wiring

I BELIEVE it is Trailbeast hosted this:

http://www.letsgocomputers.com/docs/76wiringAbody.pdf

Start with about page 9 of that pdf

The box on pass side is EGR time delay

Many times, these are emission related. I've seen distributor spark delay boxes, choke delay, EGR control...
Wait until you look under the dash. In 1974, FEDERALLY mandated seat belt interlocks were installed! If the seat belt was not connected, the car would not even spin the starter. If you had even 25 lbs sitting on the passenger front seat with the seat belt loose, the car would NOT start. Under the dash was a blue plastic control box, inside was a mini computer type circuit board. All sorts of wires ran in and out of it. In the engine compartment next to the spark ECU was a reset box for the system.
What a POS.

Yeah from 73 up they added more emissions crap year after year. That thingy in the speedo cable was emissions or economy related also. I forget did it turn on a lamp in the dash at a certain speed or what exactly. Maybe someone can link a earlier thread of its discussion.
I think the seatbelt interlock that would disable start was one year only, 1974 model. 75 and later was limited to seatbelt warning lamp. Good luck with it.

It looks like your system is fairly true to the '75 FSM. It shows the EGR delay in your pic. They show a speed switch on passenger fender that worked along with a 'throttle position solenoid' on the carb. Would love to see more pics of other areas. What is the build date on this car (from driver door)?
My late 75 build (August)had a spot to attach a box on the cowl where yours is, but no sign that anything was missing from the wiring harness. I do have a 3 wire connector that would go to one of these magic boxes (guessing the speed switch) near the AC drier in middle of passenger side inner fender. I'm sure mine did not have the seat belt interlock either. Info on the seatbelt interlock system is detailed in the Body Service Manuals for 74 and 75. I found the 76 schematic does not have all these things, and is very close to my actual wiring.
The thing in your speedo cable is also part of the EGR system. It is a 15,000 mile counter to trip the EGR lamp as a reminder to do service checks on the system. There is a screw on top to reset it. (also from the 75 manual)

Wow! Thank you all, that is some very good information. Sounds like a lot of stuff I'm probably going to be bypassing. Thanks I don't know what the build date is on the car and I'm not going to be able to make it back out to it until Sunday with my work schedule and with where I live. The car is about 15 miles away from me, but work is 10 miles in the opposite direction. I'm currently messing with it where it is sitting. Funds are a touch low being I also just bought a 69 Chevelle basket case, but it's a friend that owns it and is letting me make payments on it. A couple weeks and it'll be mine. I sold off my Scamp back in February and have been regretting that. I found my car for sale, but it's not something I'd want anymore with what's been done to it. I found this Duster on Craigslist and when I showed up, it's someone I've known for years. Along with the slant and the manual drums, it has power steering and a three by the knee with 87K miles. Looks like it had a tow bar on it, so there's a chance some of those miles were put on it being towed behind an RV. Seems like it should be a decent car.