What radio do you have in the plane now?
Nav (or comm) radios usually either work or they don't. Something to consider is the coax connections. Degraded performance is usually an antenna connection problem, or corrosion elsewhere in the connections. The coax braid and center conductor are attached to the separate "cat whisker" antenna elements with terminals and nuts, and the center conductor often breaks at the terminal. Just install a new terminal and re-attach. It is also possible the coax connections in the tray are corroded. Pulling the radio out and putting it back in often clears the corrosion enough for it to work properly again. Is there an inline splitter for use of another receiver on the same antenna (2nd nav or glideslope)? These or their connections sometimes go bad, and if they no longer connect a 2nd radio, they should be removed.
