Was not sure where to post this. Will the 03-07 Classic taillights fit my 04 dually? Any help will be appreciated.

Yes, they will fit but you will need to mod where the bulbs clip in just a little bit.

What would have to be modded? How hard would it be to do so?

All plugs are keyed so they only fit one area.
You will need to open the keying so the bulb plugs will fit.
It will take about 5 minutes each side.

Check out my ass! I mean taillights! I put them on my truck, a buddy of mine had harnesses for the lights as well, that would be your best option if you can find the lights and harnesses. I think it looks pretty dang good too!

I have the taillights already ordered. I am having trouble finding the harnesses, I guess I will have to go to the stealer:(. Am I thinking right 04-07 taillights have four bulbs in them? Dont they have a marker bulb in them?

I saw lmctruck has the harnesses in the catalog I just got. Think they are $35 per side.

I have the taillights already ordered. I am having trouble finding the harnesses, I guess I will have to go to the stealer:(. Am I thinking right 04-07 taillights have four bulbs in them? Dont they have a marker bulb in them?

You will not need new harnesses, just mod the housings a bit.
As for the marker bulds, If I remember right you will have 1 bulb on each side of your truck that will not be used on the different lenses but I may have that ass backwards.

You will not need new harnesses, just mod the housings a bit.
As for the marker bulds, If I remember right you will have 1 bulb on each side of your truck that will not be used on the different lenses but I may have that ass backwards.

Yup, that is bassackwards, the new style fleet side lights have 4 bulbs in them, you CAN mod the light assembly, but to use the bottom marker light you will have to buy a lamp holder for the small bulb (not sure which number, I think it is a 110 style bulb) then mod the assy to use the lamp holder as well as tap into the marker light wires. MUCH easier to get the harnesses, also, if you sell the truck and want to keep the lights or whatever, much easier to just unplay and unplay.
See if you can talk to some of the techs in the back of your local stealership, they usually have crap like that laying around that they would be more than happy to get rid of for a case of beer or a few bucks, that's where I got mine at!