Wouldn't his battery tender harness that he is plugging it into already have a fuse in it?
YES. Here is the deal, and after I put up my last post I went to bed and realized I need to make sure that he understands in which order the connections need to be made;
This is how you would have the heated jacket alone:
Battery---Heated Jacket Harness with fuse inline already (15 amps I think)---Heated Jacket
This is how you would have the Battery Tender :
Battery---Battery Tender Harness with fuse inline already (7 amps I think, does anyone remember)---Battery Tender
Now if you want to make ONLY one connection at the battery and use both the Heated Jacket and a Battery Tender:
Battery---Heated Jacket Harness with inline fuse---Battery Tender Harness with fuse INLINE (7 amps I think)---Battery Tender
The point is, I'm pretty sure the Battery Tender will use a smaller fuse than the heated jacket, so the Battery Tender harness needs to be the one that can be removed easily, otherwise the fuse will be seen/used, while the Heated Jacket is on and if using more than the just the jacket, the fuse will blow.
And ST11ray, the cord you showed is another good source, however, it still needs a fuse inline and I doubt that store will add one.