Before doing anything, make sure the trans is in neutral and parking brake set. Even better to elevate all drive wheels in case the engine cranks unexpectedly. Before starting the engine, be sure to have a secondary way to stop then engine in case it runs away. ( stock STOP lever that works / board you can place over the air intake , fuel you can shut off. ) Engine run away is usually caused by a fuel rack that is stuck due to gum and such.

Early Rangers use a RF fender mounted starter solenoid and a starter that does not have solenoid trigger wire. ( RE: Put power to an early Ford starter and it will engage and spin )

The wiring from the fender solenoid to the ignition switch isn't strong enough to operate the MB starter you have so it can't be used directly.

To make this work:

From the positive battery terminal, run a large cable to one of the large threaded fender mounted solenoid terminals. You will also place all of the Ranger power feeds to this terminal as well ( Both of these would be the stock Ranger setup )

Make sure the small push on wire is installed on the fender mounted solenoid. ( stock Ranger setup )

From this same fender mounted solenoid terminal, run a large cable to the large threaded terminal on the starter that isn't attached to anything else. ( You can use the Ranger cable but you will need to drill out / replace the lug on the starter end. Also, the fender solenoid end of this cable used to be on the other fender solenoid terminal. )

From the empty large threaded fender solenoid terminal, run a 10 gauge or larger ( 8 would be good. ) to the small starter mounted solenoid terminal. There might be 2 small terminals if the solenoid is set up for a gas engine. Generally, the one you want has a screw / bolt / nut, the one you don't want generally is a push on spade.

How it works:

The above setup is the same as a later model Ranger with a starter mounted solenoid.

Battery power is available at the body feed and MB starter solenoid.

When you turn the ignition switch to crank, the small wire triggers the fender starter solenoid and then it feeds power to the MB starter trigger wire. When you trigger the MB starter solenoid, it slowly rotates the starter motor and engages the drive.< Critical, see note Once the drive is engaged, the MB solenoid applies full power to the starter motor and then engine cranks.

< Note. In the initial stages of cranking, the trigger wire on a starter mounted solenoid powers up the pull in windings and rotates the drive gear to assist engagement. This takes lots of power and will burn down a small or worn out ignition switch. It can also lead to a no crank situation especially when hot.

The system hookup I've outlined above keeps this high electrical load off the ignition switch.

It is the same setup as a hot start kit sometimes found on air cooled VW engines and other brands.