Sorry for the lack of updates I haven't really done much work on the Pathfinder.
I love how the 18 sounds but it wasn't really enough output for me so I was rechecking my design and apparently my tuning was at 26 instead of 27. Another issue was my box was small so my sub was pretty inefficient, and I couldn't go any higher on my amp without heavy voltage drop and clipping. I knocked out one of the port pieces on the inside which brought my net volume from 5.5 up to ~6.4 cubes, and brought my tuning from 26 up to ~31hz. I didn't really lose any low end but it's much louder now. I gained about 5hz of extra high end which is nice.
One of these days I'll be making a video, maybe I'll do one tomorrow. I don't know if I mentioned it or not but I don't have a ton of exterior flexing because a lot of stuff on the inside flexes. I think my door panels take the brunt of the flex so my exterior panels don't really get hit with any of it. However it is loud, especially considering the sub isn't made for crazy output. I haven't hit it with a meter but I'm gonna guess it's doing low to mid 140's. If I brought my tuning up even further it would be a bit higher obviously but I'm not chasing for numbers and I like how it sounds so I think it will stay as it is.
Next step in the build is building a false floor and saving up for a second 18. I'm going to get 2 18's first and run them off one 2500. Because I don't know how long this vehicle is going to last me I'm not really willing in investing hundreds for a H.O. alt, thus I want to make my setup as efficient as possible. This may mean selling the 2500 and upgrading to a 3- 3.5k amp that is efficient and putting in a couple big batteries. Haven't decided yet, so if you guys have any suggestions that would be cool