Not what you want to hear but the heater core is buried under the dash. So much so that you strip the dash out of the car and proceed to remove the entire heater box assembly before disassembling it to get to the heater core. It is almost as if they layed the core on the assembly line and built a car around it. There is the possibility that the core itself is not leaking but instead the o-rings that seal the attaching pipes. However it would be foolish to not go ahead and install a new core as you still have huge labor just to get to the o-rings.

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90 300TE 4-M
Turbo 103, T3/T04E 50 trim
T04B cover .60 AR
Stage 3 turbine .63 AR
A2W I/C, 40 LB/HR
MS2E, 60-2 Direct Coil Control
3" Exh, AEM W/B O2
Underdrive Alt. and P/S Pulleys,
Vented Rear Discs, .034 Booster.
3.07 diffs 1st Gear Start

90 300CE 104.980
Milled & ported head, 10.3:1 compression
197° intake cam w/20° advancer
Tuned CIS ECU
4° ignition advance
PCS TCM2000, built 722.6
600W networked suction fan
Sportline sway bars
V8 rear subframe, Quaife ATB 3.06 diff