Here's how I ended up installing the rear door OEM ladder: I used M6 aluminum ribbed rivnuts and 22mm M6 stainless button head bolts. The rivnuts required a 10mm drill (you could also use 1/4-20 rivnuts - very similar in size). I already had a rivnut tool (believe it or not) so I just needed a 6mm mandrel for it. All these parts were purchased from Mcmaster Carr. Slid ladder on door from the side, made sure it looked centered and leveled, then marked the holes with a silver paint marker (truck is black). Used a punch and hammer to tap some starter dents for the drill, and used a 1/8" drill for two initial pilot holes on top of door first. Then drilled them out to 10mm. Painted the edges of holes with touchup paint, let it dry. Coated rivnuts in clear silicone sealant, and set them with the rivnut tool. Silicone oughta prevent any tendency for water to leak around the rivnuts. Put ladder back on door, loosely screwed it down, and rechecked my marks for the bottom holes - looked good so repeated the drilling/painting rivnut process for these two holes. I greased the Stainless bolts (to prevent galvanic corrosion, leaking water, etc before inserting into rivnuts and tightening down. Worked great!