A 40 amp ignition fuse fried in my 1991 Dodge Dakota. I replaced the fuse, the starter motor and the coil. The fuse fried again. What other things need to be replaced so that my engine can start?

SOURCE: 1991 dodge ram 250 van 318 engine won't start

ALWAYS CHECK THE GROUND ON A DODGE

SOURCE: 92 dodge dakota wont start

I would start with the Starter check for voltage getting to the starter should read 12 volts at both terminals

SOURCE: ignition switch fuse terminal is blowing red 40 amp fuse when i try to replace it

A fuse that is consistently blowing is telling you that there is a short in that system. Don't attempt to bridge it or use a larger amp fuse or you will have a major problem. You will need to start tracing wiring to destinations to find the short. If while moving harnesses around problem ceases to exist, look at nearby items where you are working and try to locate what was grounding. Sometimes people will stop if short disappears, leaving open the possibility of it happening again.
If you are not comfortable working with wiring, then leave it to professionals who do this every day. It's not really difficult but requires patience and a good bit of knowledge to do this right!

SOURCE: 2002 Sebring LXi Convertible Starter Motor Fuse Blown 2x

Starter solenoid. I don't have the page here but in chiltons a page shows the ignition to fuse to relay to starter and if you look closely it's quite basic. I had same problem with constant fuse blow #8. The solenoid over time wears and sometimes sticks. When it sticks, your system can overload thus burn fuse. I bet you never stalled out while driving, but only when you shut off and start somewhere it's a no go. That's because you're system is already engaged. I had to change starter as solenoid part of it all. I see others suffer this same problem so please share this around. Solenoid can stick for a millisecond.

SOURCE: 20 amp ignition fuse keeps blowing

I'm curious, does this vehicle have an alarm/remote starter? If so I expect the installer didn't wire it correctly ... it was an all too frequent issue, with inexperienced installers we had .... I would first suspect any add-on device first (if any) ... In answer to your question ... I know it does have a relay (Starter solenoid is a relay and built onto the starter) As far as it being a relay or the IGN switch ... I'm skeptical.