Now that I've told you to stay on the trail, you will need to step off a bit to locate the cache. The hiding spot is visible from the trail, the cache is not. You are looking for a copper "tack" hanging in suspension. The hide is winter friendly. Maybe.
Here are my notes about the trail from my Haystack Hide cache (GC2HWT8), assuming you are heading for the top of the mountain:
The trail to this cache is very rocky and wet. Stay on the trail for this one -- bushwhacking here is tough. The trail wanders in a direction that seems off course, but it gets you there.
From Route 9, take Haystack Road. Take the second left onto Chimney Hill Road. Turn right onto Binney Brook Road. This winds steeply up. At the stop sign, turn right onto Upper Dam Road and bear left at the next intersection. The trail head (see parking coordinates) is on your right, marked with a Haystack Trail sign.
Begin the hike on an old, rocky road. After about half a mile (Waypoint 1), turn left off the road. Do not continue to the reservoir. There's a micro cache (Needle in a Haystack, GC2HWV3) along this section to get you started. Follow the trail as it heads south and then begins to climb onto the ridge, eventually heading northward.
The GMNF states that the trail is 4.8 miles round trip and averages 3-4 hours. I usually allow about two hours, round-trip and my measurements said it was about 4 miles. Use your own judgement to determine how long it will take you. Allow enough time, wear waterproof hiking boots, prepare for icy trails if it's cold and stay on the trail.