Water is getting behind my exterior trim where it meets the foundation. What's the fix?

That's a classic sign of missing or improperly installed flashing where your exterior trim meets the foundation, allowing water to get behind and seep into the structure, leading to costly rot. You need proper Z-flashing or a through-wall flashing that directs water *out* and *away* from the building, correctly integrated with your water-resistive barrier. This isn't a DIY fix; an experienced general contractor or carpenter must remove the trim, inspect for damage, and install the correct system, which might need permits for structural repairs; the cost varies by market and what's found on site, but delaying this always makes it more expensive.