The diagram below shows the wiring schematic for the starter relay/solenoid. Since youre working on a 93 model with a mini starter,it gets wired slightly different than a 86-91 model with a regular starter.

The positive battery cable,large 4ga starter cable (that runs to the starter) the small black /white wire with silver circuit breaker and all of the wires with fusible links connect to the large solenoid post on the left.The small wire (12ga) that also runs to the starter connects to the large solenoid post on the right.So you should now have one 4ga cable running from one large solenoid post to the starter and one 12ga wire running from the other large solenoid post to the starter.The only wire left is the 18ga Red/Blue wire.This wire gets connected to the small solenoid post at the top of the solenoid.This wire has 12volts (when the key is in the start position) & triggers the contacts to close,between the two large solenoid posts,when you turn the ignition switch to the start/crank position.When the key is released to the on/run position, this wire turns into a ground.
The neg battery cable runs from the neg battery post to the engine block, to the left
of the oil filter.A 10ga wire also runs from the neg battery post then it bolts to the fender apron.It splits from there and dead ends into a black tubular plug that connects with a matching plug on the main harness. Looks like an inline fuse holder kindof.

If you don't have power to the radio,once the solenoid and battery cables are connected properly,check fuse #6 & #11.They should be hot with the key on.
The easiest way to check the fuses & circuit is by using the following method:
Attach the clip of a test light to a good ground,turn the key on,then touch the test light probe to both of the metal test pins visable at each fuse ,where you put your finger against it to push it into the fusebox. If the test light comes on at both fuse test pins , on both fuses,that indicates youve got power to both fuses and theyre not blown,so the problem either lies in the wiring between the fuses and the radio/amplifier or in the radio/amplifier itself.
If the test light fails to come on at one of the fuse test pins ,the fuse is blown.
If the test light fails to come on at both of the test pins ,the radio portion of the ignition switch is probably bad because thats what sends power to the radio fuses.

This info and the diagram below should get you fixed up.
*** BTW- if your sisters car has a stock 75amp alternator,ignore the 4ga red wire & 150amp fuse shown in the diagram below. Its used for a 3G alternator.If she converted to a 3G alternator ,the red wire and 150amp fuse are relevant. ***




OEM Neg battery cable with included pigtail.The small wire bolts to the fender apron & the black tubular plug connects to a matching plug at the end of the main engine harness.This small wire is the EEC ground & if left disconnected,youll have driveability issues or the engine will stall once the key is released to the run position.This is the 3rd most important ground in the circuit and the one that completes the ground loop,so dont forget it.

92-95 Mustang starter relay/solenoid wiring diagram