The firewall thread was for a later model with dual firewalls..

..but , now that you have jumper to x27, you have to test battery voltage..if low, then the starter solonoid will not pull in. The x27 test is from x27 downstream to starter , but it does not test the starter circuit for low V, it test power to x27.
Don't assume the starter bad until you verify Bat V
An easy test is to put the headlamps ON and watch that they do not dim out when you jumper to x27..if YES, then you have low voltage and should charge the battery before buying a starter [ and check the connections at starter, specially the main cable..

You can also put a test lamp on x27 and try the key to verify upstream.....you can see that on the schematic.
X27 is a comvienent test point we use to seperate a fault from the cabin or the engine compartment..you then know upstream/downstream circuit fault direction/isolation..a good central test point in the start circuit.

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A Dalton