Klaus;

Fuses blow because the total current flowing through them (the sum of all the downstream loads on that circuit, plus a fault current) has exceeded their rating...once it opened to clear the fault, your OD (and all the other Fuse 1 supplied loads, listed on little card inside FB cover) became nonfunctional...no surprise there...what does surprise me is why you would ask for direction, yet immediately decide you need to remove the IGN Swtich to do an exploratory disassembly???

What did the blown fuse look like...entire conductor vaporized or a slight open at a weak area?...blow fuses have information to give!

I suggest you leave your Ign Sw alone, install a modern, cheap and easily replaceable 25A fuse in one of the recommended ways, and note all associated symptoms if, and when it blows again...like when turning ON lighter or OD, and report back.

Incidentally, if I were to install an OD, I'd install a dedicated Fuse to protect this circuit...simply adding its significant load current to the Fuse 1 circuit might very well be the cause of the entire issue.

Finally, after you get your electrical situation under control, I have 25A BGF fuses available. See: http://www.sw-em.com/swemkits.htm

Good Hunting!