Snap!! ( I'm 68 ) ---so that's two of us who are "Young" Misters then.....Hooray !!....But to get back to your solenoid problems}--- In its' simplest form you will find 2 thick wires attached on the outside ( usually by a nut and spring washer )...That's the main starter lead coming FROM the battery ( these days it's usually from the POSitive terminal ) ... and then it goes from the solenoid away TO the starter motor...Those two bolts go down inside the solenoid to form contacts for engine starting current ......
Next there will be two thinner leads which normally appear to be coming from within the solenoid itself..These are the "low tension" cables and electrically are part of the ignition key / fuse / earth return circuit such that when the ignition is turned on, and you select the "START" option ( key turn or button push ) that circuit is completed and the solenoid's operating coils are energised.... The solenoid's coils are wrapped around a movable iron core which in turn has a thick piece of copper plate or bar attached across its' top.... Here comes the techy bit}----when the coils are energised they create an electro-magnetic field which causes the iron core along with the attached copper plate to move away from its resting position and make contact with those two bolts I mentioned earlier..And the starter will then turn over...........When you release the starting key or button that low tension current ceases to flow, the electro-magnetic field collapses and an internal spring pushes the copper plate/iron core assembly back to its' original resting position.......
Now you may find that there are more than two of those "Low tension" cables fitted and that they are attached to one of those thick cables ??............This is usually the vehicle designer making use of the fact that, as mentioned, one of those thick cales comes from the battery itself...Now if the battery is a long way from where an electrical supply is needed --- ( the old Mk1 Mini and MGB GT had them in THE BOOT !!) then it makes sense to take your supply from the nearest possible source ie}--the solenoid...........Your idea of a 1- for - 1 change is spot-on............I shall now retire to a darkened room --LoL !!
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