You didn't say what model year -- it really makes a difference, meaning two different probable solutions.

E.g., my '84 (and all of my earlier cars) had very loud horns, so if yours doesn't, I'd suspect that your horns, or their electrical connections or grounds, were badly corroded.

On the other hand, my two '93 240s have very weak horns, and I've heard (from many others on this forum) that it's typical of these later cars -- my solution was to install a set of four(!) horns driven by 10-ga. (and fused, of course) wiring (from the battery's junction box) and controlled through a relay switched by the original wires (near the original horn). I'm not on my home PC now, but I've got a picture I'll upload later tonight for you.