Based on your problem with the wipers it sounds like you are working in the front fuse box (a.k.a. IPM). Fuse #18 in the this box is not key-switched. Fuse #18 in the rear fuse box (a.k.a. PDC), near the battery, can be key-switched depending on the position it is inserted into the fuse block.
You have not mentioned the wattage or current rating of the amplifier. If it's worth the trouble of installing it should be a couple of hundred watts which will draw something in the range of 20 amps for a nominal 240 watts of amp power. If you connected to the load side of rear Fuse #18 (which is 20A), along with one or two other factory connected devices (like the cigar lighter, wipers, etc.), the fuse would blow due to being overloaded. If you connected to the source side of the fuse #18, and the amp draws more than 30A (as a 350-watt amp could,) you could damage other circuit components like the delay relay, or even the wiring.
In any case, an amp drawing more than 5A should probably not be connected to any existing circuit as there may be more than one OEM device already supplied by the fuse protecting that circuit, and the fuse will simply blow due to being overloaded.
To add an amp of multiple hundreds of watts it needs to be powered by an auxiliary relay straight from the battery via a wire of sufficient gage so as not to drop voltage or overheat and burn. The coil of the relay (which draws only milliamps of current) is connected to the key-switched circuit so that the relay picks up when the key is switched on (thus powering the amp) and drops out when the key is switched off (thus unpowering the amp).
As for your wiper and other problems, take another careful look around at the fuses.
2005 300C BRILLIANT BLACK, DRL, Mesh Grill/Vents, Chrome Pillars & Fender Trim, MDS Indicator, Quad Brake Lights, Tinted Windows, De-Molded, K&N Aircharger, 180 T-Stat, Flashing Sidemarkers, Predator Cust. Perf. 93 CAI Tune
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