I've just finished a self-taught crash course in antenna reception, so here's some suggestions.
Get another antenna, a standard manual pull up is fine, and connect, making sure the outer casing (not the mast) is grounded. If the reception improves, the problem is probably in the antenna or the connection of the lead to it.
While the lead is disconnected from the antenna itself, use a multitester on OHMs setting, touch the tiny center pin and the outer casing on the antenna (if the lead is a stock screw-on design). There should be no continuity. If there is, something has shorted the mast to the inner lead wire connection, and this will destroy the reception, especially AM.
Do the same to the lead (inner socket or pin to outer casing/shielding), and also check that the center wire in the coax isn't grounded, and that the outer shielding is.
Check that there is continuity in the lead itself, from the antenna end to the radio end, both outer and inner wire/shielding.
If none of these tests reveal a problem, it's probably the radio itself, or maybe someone has changed the antenna and it doesn't work very well with the stock radio.

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1986 560SL
2002 Toyota Camry
1993 Lexus