where abouts are you crossing? Sarnia or windsor? We emailed Dave Hunter that writes the book Along i-75 and he sent me an email with the directions. here is the email, I hope this isn't against the rules to post but here it is.
Regarding the construction, there is good coverage and coloured maps at the following website to guide you around it . . .
http://www.michigan.gov/documents/mdot/MDOT_Ambassador_Gateway_brochure_219274_7.pdf
1. From Sarnia/Port Huron, take I-69 West / I-94 West, towards Detroit (approx. 45 miles / 72 kms).
2. I-69 and I-94 split in three miles - continue following I-94 southwards towards Detroit (to confirm you are on the right track after the split, your next exit should be number 269 - Dove St/Range Road).
3. At I-94 exit 229, take the ramp to I-696 West, towards Lansing - also known as the "Walter Reuther Freeway" (approx. 30 miles / 48 km)
4. At I-696 exit 1, take ramp to I-275 South, towards Detroit Metropolitan Airport. You will pass the airport on your left (east) . Continue on towards I-75S (approx. 37 miles / 60 kms).
5. At the south end of I-275 (no exit number), it joins I-75 South, at I-75 mile marker 20.
You will be well below Detroit at this point, heading towards Toledo and joining my I-75 map on page 8.
Safe Travels,
Dave