They can both make it fall on it's face... you not listening? The speed signal is required for the ECT to calculate when to shift. The knock sensor error I've already explained in detail.
It's more common for the knock sensor's wiring to degrade than for the sensor's to go bad. A rewire using radio grade coax is common and recommended... from the sound of it the previous owner may have tried that and either did it wrong or poorly OR the knock sensor is indeed bad. How much the speedo cable will cost to replace depends on which segment and where you buy the replacement. The lower segment should be under $25.
Are you doing your own work or paying a shop?
If you're on a budget and don't expect that to change or if you need a reliable daily driver or you can't do your own work then these probably aren't the cars for you.
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