I love capers in my tartar sauce too. This is the recipe I make. It's delicious and I don't just use it for fish. It's good on hamburgers, hot dogs etc.
1 small shallot or onion, whichever you prefer
6 cornichons (gherkins)
1 tablespoon capers
1 cup mayo
Bunch flat leaf parsley--sometimes I leave this out, personal preference.
juice of one lemon
2 Tbsp sour cream
2 tsp mustard
1 Tbsp dill weed
salt & pepper
***** Please note, when making this, taste as you go along. You may wish to add more of lemon, mustard, dill, salt etc according to your taste.