There is a small terminal on the starter relay. It may be marked 'S', it should have a Red/Light Blue wire on it. Turning the key to 'Start', with the transmission in park or neutral is supposed to apply +12V to this terminal.
You can test this with a voltmeter, or you could just jump +12V to it.
Voltmeter Test
Check for +12V on that terminal with the key in START. If you get +12V, the problem is the starter relay itself or the wiring to the starter. If you do NOT get +12V, your problem is the going to be upstream, the ignition switch, neutral start switch, something like that.
Jump Test
Apply +12V to that terminal, doesn't matter what the key or transmission is set to, but for safety, make sure it's in park or neutral because you are bypassing that safety switch. If it does not crank, the problem is the starter relay itself or the wiring to the starter. If it does crank, your problem is the going to be upstream, the ignition switch, neutral start switch, something like that.
There's an oddity, it's rare but it's worth mentioning. The starter relay, the metal mounting must be well grounded for it to work. It is possible through some fluke that the fender isn't grounded very well.
I am giving you the wiring diagram for this section, it may help.
BUT... your profile says you have a '99 and everything I have written is for the earlier generation, so the colors may not be right, there may be something I don't know about. If you have anti-theft, that is another complication. Well, perhaps this will lead you in the right direction anyway. Good Luck!
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