Nice! Yea diesels will run forever. Just like everyone else said, routine
maintenance is key to any engine. Also may want to look into trans fluid, its condition etc. Manuals tend to be more durable then autos in a lot of cases, under normal conditions. Usually if somethin goes, its a clutch. Good fluid will extend the life of that.
Far as the speakers, I would start by pulling off the grills and taking the speakers out. First and foremost if its both the rears, or both the fronts or both the right or left...make sure the radio simply isnt panned one way (left/right) or faded one way (front to rear), be surprised that people do make that mistake. If thats all good, pull off the grills and take the speakers out, and pull radio out.Make sure the wires are connected obviously on both and connections look good. If that all looks good and the speakers still got no sound, do this...verify which wires at the back of the radio go to which speaker so u know u have the + n - to that speaker. IF you have a multi-meter/fluke meter set it to the lil "beep" function circuit tester. Touch the prongs to the speaker wires near the radio and if it beeps you have a full circuit through speaker and back. If it DOESNT and you are sure you have the right wires, wire is probably damaged somewhere in between, even a blown speaker coil will still normally complete enough of a circuit to beep by doing this. If you are sure u got the right wires and you got no beep from circuit tester, then most likely the output channels on the radio are blown. Need new one.
IF you do not have a circuit tester, you can strip the ends of the wire by the radio a lil bit, and wrap one to the + and one touch one to the - of a lil 9v battery. If there is a complete circuit u will normally hear a little click from the speaker, and better yet if you can see the speaker the cone will either move out or in a little bit depending on polarity you have it hooked up as (not important). If you have factory radio and pig tail connectors and you dont wanna rip the wires out or cut them, just slice the insulation enough to slide the insulation back 1/8" of an inch and "T" in a scrap piece of wire (like 14-18gauge'ish) to hook to the 9v battery, thus not needing to actually cut the radio wiring. And with the above, if you have pig tails, just un hook them, locates the pin or holes for the wires to that speaker and just stick the prongs against it (or in it). If you knew all this I apologize for the long read, just spent way to many years putting money into car audio, LOL. If you don't and you'd like more info...pm me.